You could use portal-filtering, but you need to keep in mind that that's only for displaying in a portal and will not influence the outcome of a sum() over a relationship.
If you need filtered results in calculations in scripts, you may want to look at one of the SQL-plugins. With these you can query FM and there you would have conditional relationships at your disposal, you can even query even totally disregarding the ERD you've defined with your relationships in FM. Useable SQL-plugin are available from several plugin-developers: myFmButler, DracoVentions, 360Works to mention a few. I use this quite often (but only in scripts, not in calculated fields)
Yes... portal filtering is a good tip - hummmm.....
UPDATE I figured out I can do OR relationships based on making one of the match field include a carrige return between two values.
Yep that's a multikey relationship, however that is not really an OR relationship:
In the RSG you can define relationship using more than one field, but then the condition on all the field must be met. A true OR relationship would more behave like a join with the same tables based on different fields.
With the multikey you have created a join based on the same fields.
But if this works for you it is okay, isn't it? :-)
have fun ;-) regards, Menno