In some protocols, a certain period of time needs to pass before a delegator's stake starts earning rewards. This period is set by the protocol and cannot be influenced by the validator.
If marked 'Custom' (e.g. Skale), the activation period depends on when the delegator submitted the delegation transaction. In the Skale network, a new epoch begins on the first of every calendar month, so if a delegator submits his/her transaction e.g. on the 15th, he/she needs to wait ~two weeks until his/her stake starts earning rewards.