Hi Boyko,
You can drag an assign to the screen and open the expression window and then you can explore what is under the folder Built-in functions:
By selecting a function you will have its description on the right pane.
Take also into account that, although most of the built-in functions are available both client and server side, there are a few exception, so you can repeat what I suggested above both in a client action or server action and take a look.
Another alternative is also to check OutSystems documentation here.
Kind Regards,
João