Functions can be used to achieve lots of different tasks.

ComplexFunctions
This page gives the basics about complex functions.

FunctionContext
In the context of the window in which they were run, but not always.

FunctionSynchronisation
To ensure functions finish a command before running the next.

FunctionTips
Ensuring commands are run in the correct context.

StartFunction
As soon as Fvwm is done reading the config file. Similarly the ExitFunction is run when fvwm ends.
https://www.fvwm.org/Wiki/Config/Functions/
