If a business is not registered for sales tax, then they cannot recover sales tax suffered (and they do not charge sales tax on their sales).
If they are registered, then for Paper F3 you assume that all sales tax suffered can be recovered. There are specific items for which there are special rules, but this is not examinable in Paper F3 – these rules are examined in paper F6, the tax paper.
I assume that you are watching our free lectures? They are a complete course and cover everything you need to be able to pass Paper F3 well.