The devs have mentioned it before, Flags.
The system checks for Flags that gets set on certain Milestones are reached.
Many of those flags are achievements, that's why for example, you can only reobtain the Garo gear if you've gotten the achievement for it.
The Calamity salvager checks the flags that are set by the achievements to see what you are allowed to purchase from it.
The mentor check is the same way, do you have this Commendation achievement flag, do you have the flags for completing the role quests, do you have this instance number achievement flag, that's just how they set up how things check other things in the game.