Contracting authorities/entities may lay down special conditions relating to the performance of a contract, provided that these are compatible with Community law and are indicated in the contract documentation (contract notices, contract documents, descriptive documents or supporting documents). These conditions may, in particular, concern subcontracting or seek to ensure the security of classified information and the security of supply required by the contracting authority/entity, in accordance with Articles 21, 22 and 23, or take environmental or social considerations into account.