This term does exist within the LAYA documents
Do you have a "scheme rules" document? Point (b) below looks to be what they're relying on.
8. What is covered under
the scheme
(a) We will pay benefits for treatment a person
receives while they are a member of their
scheme. We will pay benefits under the
scheme of which they were a member at
the time they received the treatment and
according to the rules and Benefit Table of
the scheme that applied to them at that time.
We will not pay benefits for treatment which
a person receives while he or she is not a
member of the schemes.
(b) We will only pay fees and charges for
treatment, services and facilities that are
reasonable and customary and in any event
only up to the limits shown in the Benefit
Table. By reasonable and customary we
mean that what you are charged for and how
much you are charged is not more than what
the majority of our other members of the
schemes are charged in Ireland for similar
treatment services or facilities.