One of these applies:
- this area is owned by a climbing organization (KBF, BAC, ...) and there is some kind of access regulation for it - mostly "you have to belong to one of these clubs,"
- this area is owned by someone or some company and they don't mind if you climb there,
- this area is owned by someone or some company and they have become angry at misbehaving climbers somehwere in the past and have closed the area,
- this area is owned by someone or some company but nobody really knows who and the area is remote or rarely climbed, so nobody cares. Yet.
Additionaly, most area's are Natura 2000 protected, which can mean anything, but most of the time that it's closed to climbers.
The first option is your safest bet for having a nice (but maybe crowded) climbing trip.
Be sure to check and double check your information so that you avoid the closed areas. All areas, legal and illegal, are listed here on theCrag, but access information may not be up to date.