The different categories will be laid out in your ops manual. Extract from ours:-
(a) Category A (unrestricted). An aerodrome which satisfies all of the
following requirements:
(i) an approved Instrument approach procedure;
(ii) at least one runway with no performance limited procedure
for take-off and/or landing;
(iii) published circling minima not higher than 1000 ft agl; and
(iv) night operations capability.
(b) Category B. An aerodrome which does not satisfy the Category
A requirements or which requires extra considerations such as:
(i) non-standard approach aids and/or patterns; or
(ii) unusual local weather conditions; or
(iii) unusual characteristics or performance limitations; or
(iv) any other relevant considerations including obstructions,
physical layout, lighting etc.
(c) Category B**. There are special needs for a Category B** airfield
that fall between the Cat B and C requirements. These are
airfields that have terrain, ATC or other environmental
considerations such that the company has specified particular
requirements, but that does not have to satisfy the recency
requirements of a Category C airfield.
(d) Category C. An aerodrome that requires additional
considerations to a Category B** aerodrome. Category C airfields
require the Commander to prove aerodrome competence within
the previous 12 month validity period as indicated by a valid
Category C airfield certificate.