Prior to 4.616 the entire window area of DP4 maps and controls, including frames and title bars were 'client area'. From 4.616 these areas, at least for static and edit controls, and the map frame/title itself, are optionally created as a genuine non client area. (However the area is painted entirely by the terminal manager - the windows do not have any frame or title as far as Windows itself is concerned). The primary reason this has been done is to simplify the code which is responsible for painting controls, and to make it simpler to add future 'non client' functionality such as the 'Close (X) button' or Window (system) menus. There is little or no impact on performance whether or not there is a non client area, and indeed virtually no difference in the code called by the terminal manager (there will be marginally more drawing if you disable the non client area, as it will get painted much more often, but this is partially offset by a reduction in the number of messages)
It will make it easier for an application to create its own controls in a window as the coordinates will be easier to calculate. (We've no idea what would happen if you actually tried this).
If necessary you can disable the non client area of DP4 maps by specifying usenonclient=0 in the DP4 configuration file in the [trmw] or [progname] sections.
You are recommended not to do this unless it really is necessary.