As defined in the following link to CAAP
fixed fuel reserve means an amount of fuel, expressed as a
period of time holding at 1 500 feet above an aerodrome at
standard atmospheric conditions, that may be used for
unplanned manoeuvring in the vicinity of the aerodrome at
which it is proposed to land, and that would normally be
retained in the aircraft until the final landing.
CAAP 234-1 Guidelines for aircraft fuel requirements