You would use
* for all products exclusive to any shop
= for all non-exclusive products
a shopID for all products exclusive to that shop
To search for different criteria in the same field, add a new request. Combining this:
# on a shop layout
Set Variable [ $shopID ; Shop::id ]
Go to Layout [ Products ]
Enter Find Mode
Set Field [ Products::exclusiveTo ; "=" ]
Set Field [ Products::exclusiveTo ; $shopID ]
You could also do this without a find by using a filtered relationship / portal.
I did not realise that adding a new record in Find Mode was the way to get multiple criteria. I have read my training manual and consulted the help system many times, but I tend to see what I want to see... old dog new tricks.
Your resonse is gold.