DockMode

How a control is positioned relative to its container --- attached to one edge, filling the whole client area, or not docked at all (positioned absolutely by Left / Top / Width / Height). Used by the Dock property that every concrete custom control inherits.

Constant	Value	Description
tbDockNone	0	Not docked. The control's Left, Top, Width, and Height are used directly, modulated by the control's Anchors when the container resizes.
tbDockLeft	1	Attached to the container's left edge. Width is preserved; height is stretched to the container's client area.
tbDockTop	2	Attached to the container's top edge. Height is preserved; width is stretched.
tbDockRight	3	Attached to the container's right edge. Width is preserved; height is stretched.
tbDockBottom	4	Attached to the container's bottom edge. Height is preserved; width is stretched.
tbDockFill	5	Fills the entire remaining client area, after other docked siblings have claimed their edges.

Order matters when more than one sibling is docked inside the same container: each docked control claims its edge from whatever client area remains after its earlier-added siblings have claimed theirs. The control with Dock = tbDockFill is therefore added last so that it inherits the residual space:

Private Sub Form_Load()
    lblHeader.Dock = tbDockTop      ' attached to the top, full width
    lblStatus.Dock = tbDockBottom   ' attached to the bottom, full width
    pnlTree.Dock   = tbDockLeft     ' attached to the left, between header and status
    pnlAside.Dock  = tbDockRight    ' attached to the right, between header and status
    pnlMain.Dock   = tbDockFill     ' fills whatever is left in the middle
End Sub

Setting Dock to anything other than tbDockNone makes the control's own Anchors irrelevant --- docking takes over the position and size completely. Manual positioning resumes when Dock is reset to tbDockNone.