Decision support and decision making systems find increasingly more applications in various modes of transport. In this connection, criteria of movement safety assessment must be established for manned, unmanned, and autonomous vehicles, bearing in mind the specifics of each transport mode. Distance is one of the basic criteria. In sea transport these include the closest point of approach and ship's domain understood as an area that should remain free of other objects. The authors examine the relationship between the declared domain and the actually maintained effective domain in a restricted area. They define the coefficient describing this relationship for different ship's relative bearings in the form of a mathematical function. The formulated relationship allows determining an effective passing distance based on the declared, i.e., assumed passing distance. The determined relationship also enables identification of the declarative domain based on the effective one. These relationships may be used in decision support systems of manned ships, in remote ship control centres and decision-making systems of autonomous ships for the assessment of ship movement safety, planning collision avoidance manoeuvres and generation of safe trajectories.