Aiports are put in one of three ctaegory by the operators as a result of a risk assessment of the operations at this airport.
This categorisation of airports is related to OPS 1.975 "Route and aerodrome competence qualification".
The current EU-OPS regulation does not mention anymore this risk assessment, but the elder JAR-OPS was stating:
ROUTE COMPETENCE TRAINING (JAR-OPS 1.975)
1. Prior to being assigned as pilot-in-command the pilot shall undergo training to ensure that he has obtained adequate knowledge of the route to be flown and of the aerodromes (including alternates), facilities and procedures to be used. The validity period is 12 calendar months.
2. Route competence training will include knowledge of:
a. terrain and minimum safe altitudes,
b. seasonal meteorological conditions,
c. meteorological, communication and air traffic facilities, services
and procedures;
d. for aerodrome competence qualification the operator shall:
*For Category A aerodromes; The captain is responsible for acquiring the knowledge of that aerodrome
*For Category B aerodromes; The captain should be briefed, or self-briefed by means of programmed instruction, on the Category B aerodrome(s) concerned and should certify that he has carried out these instructions.
*For Category C aerodromes; The captain should be briefed and visit the aerodrome as an observer and/or undertake instruction in a flight simulator approved by the Authority for that purpose. This
instruction should be certified by the operator.
e. search and rescue
f. navigational facilities associated with the route along which the flight is to take place.
I also understand that the differences between categories A, B and C are:
A Category A airfield satisfies all of the following requirements:
(a) An approved instrument approach procedure.
(b) At least on runway with no performance limited procedure for take-off and/or landing
(c) Published circling minima not higher than 1000ft
(d) Night operations capability
A Category B airfield is an airfield which does not satisfy all of the Category A airfield requirements, or which requires extra
considerations such as:
(a) Non Standard Approach aids and / or approach patterns, or
(b) Unusual local weather conditions or
(c) Unusual characteristics or performance limitations, or
(d) Any other relevant considerations including obstructions, physical layout, lighting etc.
A Category C airfield requires additional considerations to a Category B aerodrome and is considered to pose certain problems for
the approach and / or landing and / or take-off.