Functions of an operator or other object, that can be called by an Action from another object in an application.
A Function is a function of an Operator that can be triggered by a FUNCTION Action (link). Whether an operator offers functions and how they are called and should be used, is documented for each operator.
To view which Functions an object has, click right on the object and see if the menu says 'Functions...'. If it does and if you select that item, the Functions Dialog opens, in which you can view and try the Functions and copy their addresses. For more info see link.