( d ) o t h e r o p e r a t i o n a l f e a t u r e s the operational features of the issuance of ecb debt certificates can be summarised as follows   the certificates are issued in order to absorb liquidity from the market   the certificates can be issued on a regular or nonregular basis   the certificates have a maturity of less than 12 months   the certificates are issued through standard tenders ( as specified in section 51 ) 