One of the reasons why I led with Trans-Humanoid instead of Techno-Humanoid was that I thought it would be a really interesting concept to have the THMPR's core systems be biological in nature, in other words, it was once human before being turned into the THMPR. It would open up some interesting storyline options as well as the ability to explore some of the humanistic concepts.
Mobile is kinda obvious, but at the same time most drilling apparatus in today's world is actually NOT mobile, it has to be deconstructed and then moved to its new spot before being put together again. As such I felt that indicating its maneuverable status might be considered important.
Personalized, again kinda obvious. each THMPR is designed for a particular Reaper, even if it is not directly owned by said reaper it has been altered to fit their needs. This sets it apart from other corporate machines (which could also be seen in game around bases) that lack the customization options like we have with the THMPR.
Rig is actually a term used widely for such platforms, Oil Rig, Drilling Rig, etc. I felt like keeping the "nostalgia" as it were might be appropriate. It would indicate its use as something other than a terraforming or a recon/rover type unit. Remember that while it may have defensive options, and yes it does bring resources back to base, it's main purpose is mining.