If your vendor doesn't listen to your pleas to allow for a blank TIN, you could set up procedures outlining exactly what number to use for each situation. For example 999-99-9999 would be a foreign national, 888-88-8888 would be a "House Account" or the like, and so on. That way you can run reports on just those TINs and figure out what is what. Might take a little bit of investment upfront to do maintenance on the existing CIFs, but it could yield nice benefits down the road.
Just make sure that whatever numbers you end up using for these various categories are never going to be used as real TINs.
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