It could mean an airside certificate which you can’t do yourself, the firm has to put you through that and it stays in their name.
Or it could mean Level D - Aviation Security, which is for transporting air-freight, you don’t go airside but can deal with ‘known freight’.
I’ve had both, I worked airside when I was a coach driver - took the aircrew from the plane to the hotel and back - and I had aviation security since I gave up coach driving, was a van driver with Level 4, then Class 2 level 4, then class 1 level 4, then level d, then when it was due for renewal and was going to cost me money again, I did ADR instead.
Airfreight used to be an ok job, but then there was a lot of jobsworths got involved and it became a pain in the harris - basically multi-drop but with a bonding cord