To come up with a permanent user guide is a bit impossible since Shotcut is continuously evolving every few months. To read all of the changes, look here.
When cropping in Shotcut, you are cropping a selected video clip to your project’s Video Mode.
The Video Mode is set first (Resolution/FPS) before you bring any media into your project.
There are 2 documentation posts for cropping.
To see what is possible with Shotcut, look through this section: