Transaction form - hiding and showing custom fields

Firstly, an apology - I'm very new to Netsuite, and have been thrown in very much at the deep end; so I hope that this hasn't been asked a dozen times before.

I've been asked to make a couple of amendments to Netsuite transaction forms. The company works in a couple of regions, and they want a checkbox added to one region for a new product. So far, so good.

The form that's giving me the problem is the Estimate form. For most of the transaction forms, there are different versions for different regions; NetSuite displays the appropriate one according to the user's Location in their Classification. The Estimate doesn't seem to work like that - there's a single form; but the contents are still varying according to which Location you're in, and I can't for the life of me work out how it's happening.