For the benefit of those who don't know, airline staff travel is mostly "Sub load"
Sub load = Subject to load.
It means you get the empty seat because there aren't enough full fare paying passengers to fill the aircraft. It also means that if a full fare paying passenger suddenly arrives, even at the last minute, then you have lost your seat.
This means that the "Ten percent fare" isn't always the bargain it is made out to be, like when you are stuck in London for a few extra days waiting for a seat home. I spent a fascinating but unscheduled six days in Karachi as the guest of Lufthansa on one of these jaunts. The ATO once looked at these fares with respect to fringe benefits tax and decided they weren't worth the trouble of taxing.
Some airlines will sometimes give staff a confirmed seat in certain circumstances, and occasionally upgrades, for example Ansett used to give you confirmed annual leave travel once a year, with an upgrade to first class if it was available.
The infinite gradations in determining access to staff travel are one of the tools used by airline management to divide, irritate, and rule their workforces.
As an engineer, we always got the short end of the stick. It was always the cabin crew who seemed to get the "long weekend in Hawaii" type perks. Certain managers often headed overseas with half a dozen first class open dated tickets in their pockets.