One of the user options along with a cache balance transaction is to set a witness on the cached account balance. A valid witness shows that the addresses have not been updated since the last cache balance write, i.e. are still in the UTXO set. The witnesses are not handled by the core validators. They are treated as simple writes, and maintained by the users or delegated to third-party services (see delegated write transactions).
The balance witnesses are mainly relevant for when cryptographic accumulators will be used to maintain the UTXOs (as a part of the load balancing system and for stateless consensus). Currently, the witness is simply a flag and can only be verified by full nodes, i.e. any nodes who can query the UTXO set to check the UTXOs individually. Balance witnesses are used for accounts that are not updated very often.