What Rod1 describes is most normal for payments dry.
More normal in my experience however, is a wet rate, on hours/hobbs/tacho (depending upon syndicate), with local fuel charged directly to the syndicate, and receipted fuel bought elsewhere taken off the bill.
Ultimately however, no two syndicates are identical, so if you're planning to join one - look at the rules, then chat to somebody about how that syndicate actually interpret their own rules.
G