The Z order of a window indicates the window‘s position in a stack of overlapping windows. This window stack is oriented along an imaginary axis, the z-axis, extending outward from the screen. The window at the top of the Z order overlaps all other windows. The window at the bottom of the Z order is overlapped by all other windows.

Windows maintains the Z order in a single list. Windows are added to the Z order based on whether they are topmost windows, top-level windows, or child windows. A topmost window overlaps all other non-topmost windows, regardless of whether it is the active or foreground window. A topmost window has the WS_EX_TOPMOST style. All topmost windows appear in the Z order before any non-topmost windows. A child window is grouped with its parent in Z order.
When an application creates a window, Windows puts it at the top of the Z order for windows of the same type. You can use the BringWindowToTop function to bring a window to the top of the Z order for windows of the same type. You can rearrange the Z order by using the SetWindowPos and DeferWindowPos functions.

The user changes the Z order by activating a different window. Windows positions the active window at the top of the Z order for windows of the same type. When a window comes to the top of Z order, so do its child windows. You can use the GetTopWindow function to search all child windows of a parent window and return the handle of the child window that is highest in Z order. The GetNextWindow function retrieves the handle of the next or previous window in Z order.

Z Order（Copy From WIN32.HLP）