This command is found at
-> in each image menu.This editor gathers together several selection functions that you can access to by a simple click on a button:
The Select All button.
The Select None button.
The Select Invert button.
The Save to Channel button.
The To Path button.
The Stroke Selection button.
There, selected areas of the image are white, non selected areas are black and partially selected are grey leveled. Clicking on this window acts as select-by-color: if somebody can explain that blindly function...