To separate local pressure flying (near an aerodrome) from standard pressure flying (in the upper air), ICAO defines two critical altitudes:

Transition Altitude (TA) is a height above ground specified for each aerodrome. Below the TA, all aircraft use local QNH. Above the TA, all aircraft use standard pressure (1013 hPa) and refer to altitudes as Flight Levels (FL).

In Europe, the TA varies by country and aerodrome elevation. For example, many countries use 5,000 feet, while some use 3,000 feet. Higher elevations typically have higher transition altitudes. You will find the TA for each aerodrome in the AIP or on approach charts.

Transition Level (TL) is a Flight Level specified by ATC. It is the first Flight Level above the TA.

When you climb from the ground, you use local QNH until you reach the TA. At the TA, you transition to 1013 hPa and refer to your altitude as a Flight Level (FL). When you descend, you use FL until you reach the TL, then switch back to local QNH.