Transition altitude & transition level

Transition altitude: the altitude at or below which vertical position is given as an altitude (QNH).
Transition level: the lowest flight level available for use above the transition altitude (reference pressure 1013.2 hPa).
Climbing through the transition altitude: set 1013.2 hPa and report subsequent levels as flight levels.
Descending through the transition level: set QNH (or QFE if used locally) and report as altitude/height.
The transition altitude is published per aerodrome or FIR and varies by State — do not assume a fixed value.

Source: ICAO Annex 2 / SERA – Transition altitude and level

IFR