Plain English Meaning

On the chart, instead of telling you the exact level where you switch from flight levels back to altitudes, it says ATC will tell you. The controller gives you the number when you need it.

Full Definition

A notation found on instrument approach charts indicating that the transition level — the lowest flight level available for use above the transition altitude — will be assigned by Air Traffic Control rather than being fixed and published. The transition level marks the boundary above which aircraft fly using standard pressure setting (flight levels) and below which they use the local altimeter setting (altitudes in feet).