We make use of Level-capping every day. It affects primarily tfc outbound EDDF and EDDM with destinations in northen Italy for the Frankfurt departures and Lyon/Geneva for the Frankfurt and also Munich departures.
These flights are then restricted to F230 in the one direction and F220/F240 in the other direction.
This measure of flow control was introduced to relieve the upper sector which works tfc between F250 an F300.
Funny thing though is that this sector used to run at or above capacity most of the time, and now it runs well below capacity.
Most of my fellow ATCs dont understand that the requested flight level in the flight plan is not really "the requested" flight level, but the level-capping flight level. And when you phone the next sector and tell the controller that there are no restrictions for one or more tfc which you have observed cruising along well below your sector, he aks the pilot "what is your requested cruising level?" The pilot answers politely "F230" as he filled it out because apparently the ATC providers have complained to the airlines about asking for higher levels when they dont have a slot in the upper airspace.
I tell my colleagues that you should phrase the question correctly so that the pilot understands that your offering climb and not just checking details on the flight plan.
In my mind, level-capping is very restrictive and in our airspace it "punishes" our biggest customer ie. Lufthansa. But the folks here reason that the pilots should always fly at their "requested" flight levels and not be given climb to higher, more economic flight levels.
Providing a service when you have the capacity to do so is a non-entity.