Tank Recovery Vehicles
Medics, mechanics, scientists, workers, scribes - TRVs are the most versatile class.
TRVs help the machines with upgrades and repairs. They can perform tasks that other vehicles cannot do, and benefit from some protection because of that. Their fields of expertise include their main purpose, offering repairs, but also a wide range of other services like construction and road works, and handicraft. Some TRVs offer extended services - they call themselves mindsmiths, and basically do psychotherapy. Many TRVs live a nomadic lifestyle and simply wander around, usually on a route they settled on where they frequent bases on the way. They also act as traders, offering fuel and parts in exchange for equally valuable things. In some regions, they set up trading posts in neutral territories, often ones that are endorsed by clans in some way. Since travelling alone is still dangerous, some TRVs hire guards that accompany them on their travels. Others join teams. Clans have many of them at their disposal.
Civilization and Culture
TRVs in more civilized areas tend to form unions and organizations that specialize in different fields of expertise or advocate different schools of ethics and methodology. Those unions and organizations could be viewed as similar to clans, but without the territorial and militant aspects. They also exist independently from clan allegiances, which means that a TRV could be part of a union and a clan at the same time.
TRVs are bound by ethics for the most part - they have dedicated their lives to helping others. Giving up the majority of one's ability to fight is a big sacrifice.
Various famous scientists and scribes are known among the TRVs. Some of them are surrounded by controversy - infamous examples include Trevor (author of a radio essay discussing the Purge), and Graverobber (leading mechanic of the Legion). There are also some who were criminals and evil - for example a TRV known as Brun, who is remembered for scamming a big clan (which led to his execution by said clan). It is a common belief that the famous entity Argus was a TRV, although there are also many machines who belief that he was a human or deity of sorts.
Interspecies Relations and Assumptions
TRVs benefit from a protected status among vehicles. Most tanks don't go out of their way to kill them; a travelling TRV spotted by a team is usually not in danger. Their usefulness is considered more valuable than the spare parts killing them would yield. They don't carry very valuable equipment with them either - their inventory consists of cheap parts that are suitable for replacing broken equipment in a case of emergency, but inferior to anything one would loot from a team. Thus, killing a TRV is generally an act of desperation or simply ill will. A common stereotype is that all or most of them are pacifists, which is somewhat of a bad word in the Hummelverse. Also widespread is the belief that TRVs choose their trade because they are too weak in mind or too unskilled to fight. This belief is not necessarily considered pejorative and often presented as a natural disposition that they are not shamed for.