From HEXONET Wiki
The PayDomainRenewal command is used to pay the renewal of a single domain. It sets the PAIDUNTILDATE of a domain according to the given renewal period. If a domain has been paid (as given by PAIDUNTILDATE) it will be renewed automatically when registry's EXPIREDATE is reached.
(required) command = PayDomainRenewal domain = (DOMAIN) (optional) period = (INT)
(required) code = (CODE) description = (DESCRIPTION)
Please have a look at HEXONET Renewal System to understand how the system internal renewal works.