The last vacated level? IMO, it's the either (a) the last level you had been assigned and were maintaining, or (b) the level through which you were passing when you made initial contact on the new frequency following frequency change.
AIP ENR 10.3.6 is here, and reads as follows,
After any frequency change, pilots must advise the last assigned level, and if not maintaining the assigned level, the level maintaining or the last vacated level eg, “MELBOURNE CENTRE (CALLSIGN) CLEARED FLIGHT LEVEL TWO ONE ZERO, MAINTAINING FLIGHT LEVEL THREE ONE ZERO”.
However, I believe the "eg" given is a poor example, and would be better written as,
"MEL CENTRE (CALLSIGN) MAINTAINING FL310, CLEARED TO DESCEND TO FL210", and for clarification perhaps another "eg" such as, "MEL CENTRE (CALLSIGN) LEFT (or VACATED) FL310, DESCENDING TO FL210".
But I still don't get why!?
It allows the new ATC'er to confirm that the info he has received on hand-over from the previous ATC'er is correct ("Handing aircraft XYZ to you, he's cleared to FL210"), and that the aircraft pilot has received and UNDERSTOOD the
correct level to which he is cleared.
Checks and cross checks.
(But if it's an American crew, then expect to hear something like, "MEL BORN CENTRE YANKEE 69 CHECKIN' IN AT THREE ONE OH FOR TWENTY-ONE")
