The help included with EA 10.0.1005 gives the following description:
returns a value from an invoked add-in function - syntax: addin:<addin_name>, <function_name>, <parameter> [, <parameter> ...]
and the following example shape script which also describes the expected method signature in the add-in:
// GET A VALUE RETURNED BY AN ADD-IN
shape main
{
//Draw a simple rectangle
Rectangle(0,0,100,100);
//Print string value returned from Add-In "MyAddin",
//Function "MyExample" with two string parameters
Print("#ADDIN:MyAddin, MyExample, param1, param2#");
}
// METHOD SIGNATURE FOR ADD-IN FUNCTION:
// Public Function MyExample(Repository As EA.Repository,
// eaGuid As String, args As Variant) As Variant