5. Costs that pertain to both en route air navigation services and terminal air navigation services shall be allocated in compliance with the general principles referred to in Article 21(4), point (l). Cross-subsidy between en route air navigation services and terminal air navigation services shall not be allowed. Cross-subsidy shall be allowed between different air navigation services within either one of those two categories only when justified for objective reasons, subject to transparent identification in accordance with Article 36(3).