Most of these relate to the non-client (NC) region of the form. The client region of the form is the region where the normal interactive controls are placed, whereas the non-client section of a form includes the border, title bar, minimize, maximize, close and help buttons, etc. which make up the OS look and feel.

C# Definition:

/// <summary>Options available when a form is tested for mose positions.</summary>
enum WindowHitTestRegions
{
    /// <summary>HTERROR: On the screen background or on a dividing line between windows
    /// (same as HTNOWHERE, except that the DefWindowProc function produces a system
    /// beep to indicate an error).</summary>
    Error = -2,
    /// <summary>HTTRANSPARENT: In a window currently covered by another window in the
    /// same thread (the message will be sent to underlying windows in the same thread
    /// until one of them returns a code that is not HTTRANSPARENT).</summary>
    TransparentOrCovered = -1,
    /// <summary>HTNOWHERE: On the screen background or on a dividing line between
    /// windows.</summary>
    NoWhere = 0,
    /// <summary>HTCLIENT: In a client area.</summary>
    ClientArea = 1,
    /// <summary>HTCAPTION: In a title bar.</summary>
    TitleBar = 2,
    /// <summary>HTSYSMENU: In a window menu or in a Close button in a child window.</summary>
    SystemMenu = 3,
    /// <summary>HTGROWBOX: In a size box (same as HTSIZE).</summary>
    GrowBox = 4,
    /// <summary>HTMENU: In a menu.</summary>
    Menu = 5,
    /// <summary>HTHSCROLL: In a horizontal scroll bar.</summary>
    HorizontalScrollBar = 6,
    /// <summary>HTVSCROLL: In the vertical scroll bar.</summary>
    VerticalScrollBar = 7,
    /// <summary>HTMINBUTTON: In a Minimize button. </summary>
    MinimizeButton = 8,
    /// <summary>HTMAXBUTTON: In a Maximize button.</summary>
    MaximizeButton = 9,
    /// <summary>HTLEFT: In the left border of a resizable window (the user can click
    /// the mouse to resize the window horizontally).</summary>
    LeftSizeableBorder = 10,
    /// <summary>HTRIGHT: In the right border of a resizable window (the user can click
    /// the mouse to resize the window horizontally).</summary>
    RightSizeableBorder = 11,
    /// <summary>HTTOP: In the upper-horizontal border of a window.</summary>
    TopSizeableBorder = 12,
    /// <summary>HTTOPLEFT: In the upper-left corner of a window border.</summary>
    TopLeftSizeableCorner = 13,
    /// <summary>HTTOPRIGHT: In the upper-right corner of a window border.</summary>
    TopRightSizeableCorner = 14,
    /// <summary>HTBOTTOM: In the lower-horizontal border of a resizable window (the
    /// user can click the mouse to resize the window vertically).</summary>
    BottomSizeableBorder = 15,
    /// <summary>HTBOTTOMLEFT: In the lower-left corner of a border of a resizable
    /// window (the user can click the mouse to resize the window diagonally).</summary>
    BottomLeftSizeableCorner = 16,
    /// <summary>HTBOTTOMRIGHT: In the lower-right corner of a border of a resizable
    /// window (the user can click the mouse to resize the window diagonally).</summary>
    BottomRightSizeableCorner = 17,
    /// <summary>HTBORDER: In the border of a window that does not have a sizing
    /// border.</summary>
    NonSizableBorder = 18,
    /// <summary>HTOBJECT: Unknown...No Documentation Found</summary>
    Object = 19,
    /// <summary>HTCLOSE: In a Close button.</summary>
    CloseButton = 20,
    /// <summary>HTHELP: In a Help button.</summary>
    HelpButton = 21,
    /// <summary>HTSIZE: In a size box (same as HTGROWBOX). (Same as GrowBox).</summary>
    SizeBox = GrowBox,
    /// <summary>HTREDUCE: In a Minimize button. (Same as MinimizeButton).</summary>
    ReduceButton = MinimizeButton,
    /// <summary>HTZOOM: In a Maximize button. (Same as MaximizeButton).</summary>
    ZoomButton = MaximizeButton,
}

VB.NET Definition:

''' <summary>Options available when a form is tested for mose positions.</summary>
Enum WindowHitTestRegions
    ''' <summary>HTERROR: On the screen background or on a dividing line between windows
    ''' (same as HTNOWHERE, except that the DefWindowProc function produces a system
    ''' beep to indicate an error).</summary>
    [Error] = -2
    ''' <summary>HTTRANSPARENT: In a window currently covered by another window in the
    ''' same thread (the message will be sent to underlying windows in the same thread
    ''' until one of them returns a code that is not HTTRANSPARENT).</summary>
    TransparentOrCovered = -1
    ''' <summary>HTNOWHERE: On the screen background or on a dividing line between
    ''' windows.</summary>
    NoWhere = 0
    ''' <summary>HTCLIENT: In a client area.</summary>
    ClientArea = 1
    ''' <summary>HTCAPTION: In a title bar.</summary>
    TitleBar = 2
    ''' <summary>HTSYSMENU: In a window menu or in a Close button in a child window.</summary>
    SystemMenu = 3
    ''' <summary>HTGROWBOX: In a size box (same as HTSIZE).</summary>
    GrowBox = 4
    ''' <summary>HTMENU: In a menu.</summary>
    Menu = 5
    ''' <summary>HTHSCROLL: In a horizontal scroll bar.</summary>
    HorizontalScrollBar = 6
    ''' <summary>HTVSCROLL: In the vertical scroll bar.</summary>
    VerticalScrollBar = 7
    ''' <summary>HTMINBUTTON: In a Minimize button. </summary>
    MinimizeButton = 8
    ''' <summary>HTMAXBUTTON: In a Maximize button.</summary>
    MaximizeButton = 9
    ''' <summary>HTLEFT: In the left border of a resizable window (the user can click
    ''' the mouse to resize the window horizontally).</summary>
    LeftSizeableBorder = 10
    ''' <summary>HTRIGHT: In the right border of a resizable window (the user can click
    ''' the mouse to resize the window horizontally).</summary>
    RightSizeableBorder = 11
    ''' <summary>HTTOP: In the upper-horizontal border of a window.</summary>
    TopSizeableBorder = 12
    ''' <summary>HTTOPLEFT: In the upper-left corner of a window border.</summary>
    TopLeftSizeableCorner = 13
    ''' <summary>HTTOPRIGHT: In the upper-right corner of a window border.</summary>
    TopRightSizeableCorner = 14
    ''' <summary>HTBOTTOM: In the lower-horizontal border of a resizable window (the
    ''' user can click the mouse to resize the window vertically).</summary>
    BottomSizeableBorder = 15
    ''' <summary>HTBOTTOMLEFT: In the lower-left corner of a border of a resizable
    ''' window (the user can click the mouse to resize the window diagonally).</summary>
    BottomLeftSizeableCorner = 16
    ''' <summary>HTBOTTOMRIGHT: In the lower-right corner of a border of a resizable
    ''' window (the user can click the mouse to resize the window diagonally).</summary>
    BottomRightSizeableCorner = 17
    ''' <summary>HTBORDER: In the border of a window that does not have a sizing
    ''' border.</summary>
    NonSizableBorder = 18
    ''' <summary>HTOBJECT: Unknown...No Documentation Found</summary>
    [Object] = 19
    ''' <summary>HTCLOSE: In a Close button.</summary>
    CloseButton = 20
    ''' <summary>HTHELP: In a Help button.</summary>
    HelpButton = 21
    ''' <summary>HTSIZE: In a size box (same as HTGROWBOX). (Same as GrowBox).</summary>
    SizeBox = GrowBox
    ''' <summary>HTREDUCE: In a Minimize button. (Same as MinimizeButton).</summary>
    ReduceButton = MinimizeButton
    ''' <summary>HTZOOM: In a Maximize button. (Same as MaximizeButton).</summary>
    ZoomButton = MaximizeButton
End Enum