The country (well England - note sure about the rest of the UK) is split into Centres. So (for example) South Midland Centre, South Eastern Centre, etc, etc.
Some trials are totally ‘open’ so it doesn’t matter where your club is based. Most are either ‘open to centre’ (so your club has to be in that centre’, ‘group’ (so a selection of clubs in that group - the Meteor Group in the South Midland Centre is an example) or ‘closed to club’ (so just for members of that club).
You’ll see what an event is from the supplementary regs on the ACU website.
But per earlier you can join more than one club.
Hope that makes sense!