1. “Shall”:
ICAO:
Any specification for physical characteristics, configuration, matériel, performance, personnel or procedure, the uniform application of which is recognized as necessary for the safety or regularity of international air navigation and to which Contracting States will conform in accordance with the Convention; in the event of impossibility of compliance, notification to the Council is compulsory under Article 38.
EUROCAE ED-120 and ED-110B
Indicates a mandated criterion; i.e. compliance with the criterion is mandatory and no alternative may be applied.
2. “Should”:
ICAO:
Any specification for physical characteristics, configuration, matériel, performance, personnel or procedure, the uniform application of which is recognized as desirable in the interest of safety, regularity or efficiency of international air navigation, and to which Contracting States will endeavour to conform in accordance with the Convention.
EUROCAE ED-120 and ED-110B
Indicates that, although the criterion is regarded as the preferred option, alternative criteria may be applied. In such cases, alternatives should be identified in appropriate approval plans and agreement sought from the approval authority.
3. “May” means a procedure, or instruction is permissive, optional or alternative.
4. “Will” is only used for informative or descriptive writing, e.g. ”transfer of CPDLC will coincide ….” is not an instruction to the controller.
5. “Instruct” indicates that an ATM operator is required to issue binding positive commands to the addressee.
6. “Advise” means that an ATM operator is required to suggest a non-binding behaviour to the addressee;
Source:
ATC Data Link Operational Guidance for LINK2000+ Services
Edition 5.1 Edition date: 01.03.2010