Prioritary, as it appears in scholarly works, is a term from algebraic geometry for a concept I am not qualified to describe even in the vaguest terms. This sense is far from common in English, however, and does not appear in any major dictionary or in the OED.
As an adjective, priority is usually synonymous with high priority, i.e. assigned a higher degree of importance or attention: a priority client, priority boarding, a priority transmission from Starfleet, and probably, priority cities in the scope of the XYZ policy.
As a noun, however, priority can refer either to the order of precedence in general— you might triage your email inbox into high-priority, normal priority, and low-priority messages— or to the assignment of a higher order of precedence or attention to something.
Some languages appear to employ different words to make the distinction, e.g. priorité and prioritaire, приоритет and приоритетный, but English does not. Where you see prioritary in a non-mathematical context, therefore, I would suppose it to be a mistranslation.
Similarly, the verb to prioritize refers both to the general evaluation of something for its importance and to the assignment of a high level of importance to something. Thus, this filter prioritizes messages from clients can mean either that it evaluates and assigns a priority to the messages from clients, or that it assigns a high priority to all messages from clients and a lower priority to other messages. These messages are prioritized is similarly ambiguous. Additional context is required to understand which meaning is intended.