Refer to figure.
In flight, when an aircraft is at or below the transition altitude (TA), its vertical position shall be expressed in terms of altitude. The TA is the upper limit from the surface to use the local QNH altimeter setting. At the Transition Altitude (TA), when climbing, change altimeter from QNH to standard setting.

Once the en-route flight is at or above the transition level (TL), the vertical position of the aircraft shall be expressed in terms of flight levels. At the Transition Level (TL), when descending, change altimeter from standard used for flight levels to local QNH.

Finally, the airspace between the TA and the TL is called transition layer. The thickness of this layer varies from country to country.

When passing through the transition layer, the vertical position of the aircraft shall be expressed in terms of flight levels when climbing and expressed in altitude when descending.