First or second claim what does it mean?

When you join a running club, providing it is your only club, that is your first claim club. If you subsequently join another club, that will be your second claim club. You can race as an individual in open races for any club you are a member of. However, for league and championship races and to count as part of a scoring team, you have to compete for your first claim club, unless it is not affiliated to UKA for that discipline, or is not participating in that league. This system is primarily designed to stop elite athletes swapping from club to club regularly.