This is one of the weaknesses of the tribunal system.
Quite often the tribunal members do not know enough about the law, and they rely upon the CMS reps to educate them.
The answer is provided by section 441 of the CSA 1991:
''(1) The Commission shall have jurisdiction to make a maintenance calculation with respect to a person who is—
(a) a person with care;
(b) a non-resident parent; or
(c) a qualifying child,
only if that person is habitually resident in the United Kingdom, except in the case of a non-resident parent who falls within subsection (2A).
(2) Where the person with care is not an individual, subsection (1) shall have effect as if paragraph (a) were omitted.
(2A) A non-resident parent falls within this subsection if he is not habitually resident in the United Kingdom, but is—
(a) employed in the civil service of the Crown, including Her Majesty's Diplomatic Service and Her Majesty's Overseas Civil Service;
(b) a member of the naval, military or air forces of the Crown, including any person employed by an association established for the purposes of Part XI of the Reserve Forces Act 1996;
(c) employed by a company of a prescribed description registered under the Companies Act 2006; or
(d) employed by a body of a prescribed description.''
Can't help you re "prescribed description" without undertaking research I have no time to do.