the operational features of these operations can be summarised as follows   they are liquidityproviding operations   their frequency can be regular or nonregular   their maturity is not standardised a priori   they are executed through standard tenders ( as specified in section 51 )   they are executed in a decentralised manner by the national central banks   all counterparties fulfilling the general eligibility criteria ( as specified in section 21 ) may submit tender bids for structural reverse operations  and  both tierone and tiertwo assets ( as specified in chapter 6 ) are eligible as underlying assets for structural reverse operations 