A vessel in this clause refers to either a ship or aircraft. Vessels operate in both international and sovereign waters and airspace. At such times as the vessel is in international waters and airspace, the vessel's passenger communication must comply with the regulations of the territory with sovereignty over the location they are in.
At the same time, the vessel and - this is the point - the UEs operating in that vessel may be subject to regulations of the country of its registration. The regulatory context changes when the ship is in port or the aircraft is on the ground. There it is clearly the case that sovereign regulations of the territory apply. However, even in this case, there are some regulatory aspects that relate to the country of registration.
The Tokyo Convention 
states that the laws of the country of registration of the aircraft apply to acts committed on board.
The Convention on the High Seas 
defines the notion of a 'flag state' registration of the vessel. These laws apply to the passengers on board. From Article 6 "Ships shall sail under the flag of one State only and, save in exceptional cases expressly provided for in international treaties or in these articles, shall be subject to its exclusive jurisdiction on the high seas."
This implies that passenger communication requirements of the vessel in international regions are subject to the regulations of the country of registration.
A separate regime applies to vessels in sovereign waters and airspace. In this case, international law is more complex. The Tokyo Convention Article IV states "A Contracting State which is not the State of registration may not interfere with an aircraft in flight in order to exercise its criminal jurisdiction over an offence committed on board except in the following cases:
the offence has effect on the territory of such State;
the offence has been committed by or against a national or permanent resident of such State;
the offence is against the security of such State;
the offence consists of a breach of any rules or regulations relating to the flight or manoeuvre of aircraft in force in such State;
the exercise of jurisdiction is necessary to ensure the observance of any obligation of such State under a multilateral international agreement."
Thus, it is entirely possible for an aircraft to impose communication regulations of the registered state while at the same time complying with the above convention. This is analogous to the situation of a vessel in national waters.
While an aircraft is in sovereign airspace, regulations may apply to communications by passengers' UEs. For example, over the United States of America, use of mobile telecommunications using a terrestrial radio access is not permitted. 
A third important scenario is the ship in port or an aircraft that has landed. In this case the sovereign regulations of the territory apply. However, there are - for telecommunications especially - certain conditions that are necessary to consider. If communications equipment on board the vessel has been certified by their registered state, these certification requirements may apply to communications rather than those of the territory where they are in port. 
The relevance to satellite communication arises due to the distinct regulatory implications based not only on the location of the aircraft or vessel, but also its status (is it airborne? Is it at port?)
It is recommended that for UEs that are operating outside of sovereign territory, the context of the operation is taken into account, i.e. the national registration of the vessel will determine the regulatory regime applying to the UEs on the vessel.
When a vessel is operating in national waters, or sovereign airspace, the regulations of the corresponding territory apply in addition to those of the national registration of the vessel for communication by UEs in that vessel.
When a vessel is in port (i.e. a harbour or an airport), the regulations of the sovereign territory apply for communication by UEs on that vessel. The notable exception to this is that the communication equipment certification of the vessel may be those of the national registration of the vessel.
It is assumed that applicable regulations will be presented to passengers and that passengers will fully comply. From a 3GPP perspective there are no additional standards requirements to identify and comply with related to maritime-specific or airspace-specific regulations for UEs on board vessels.