This will register an action when Aneuch is receiving POST data. Each form should have a hidden field titled 'doing'. The value of that field is what will be used to determine which action in %PostingActions will be called (and %PostingActions is built using this subroutine).
Name - This is the name to associate with the action (and what the form's hidden "doing" field should have as its value).
Subroutine - This should be the subroutine that is called when the POST action you specify is called. Must be entered in this format: \&SubroutineName
These are actual examples from the Aneuch source code:
This will register the action (hidden "doing" field on the form) 'login' with the subroutine DoPostingLogin