The ALT key alters the copying of the screen (to the buffer) to be just the active window.
That is,
PrtScr copies the ENTIRE screen.
and
Alt+PrtScr copies just the top-most, the active window.
While
PrtScr will do what you want, it may be so much that you then need to crop down your captured area to just have the selected portion.
Whereas
Alt+PrtScr may be just all you want or need.
In neither case will the mouse cursor be captured. If you want to capture the cursor then you really need a tool that will do that, like
IrfanView (HIGHLY recommended!).