All certificates are issued for a restricted period of time. However it may happen that a certificate should not be used or becomes invalid before the "not after" time in the certificate is reached. In this case the issuing CA should revoke this certificate by putting it on the list of revoked certificates, signing and publishing it.
In XCA this can be done by the context-menu of the CA and the "revoke" entry in the context-menu of the issued certificate. First all invalid certificates must be marked as revoked and then a Certificate Revocation List should be created and will be stored in the database.