Can some one explain why this works?

@ismbr(RPU))

All,

Writing a BSO calc script.

I have a calculated measure RPU that is defined (MajXXXX + MajYYY)/2

When I view the calculated measure in YTD or QTD I want it to always use MTD

So I go and change the way that I view my YTD to say soemthing like

IF (@ismbr(RPU)) Jan;

Else DecYTD + Jan

Endif;

Executing the above give me JanYTD which is what I don't want.

If I change the script to:

IF(

     @ismbr(MajXXXX) or

     @ismbr(MajYYYY) ) Jan;

Else DecYTD + Jan

Endif;

When executed it works.  I am at a a loss, as when I read