What might "Volume Unit" mean, in TV credits?

"Masters of the Air" (imdb.com/title/tt2640044/fullcredits) has these subheadings in the end credits scroll:

- Cast

- Post Production

- Volume Unit

- Second Unit

Any ideas what the term "volume" refers to, in this specific context? The use of the usual "second" suggests to me that it isn't simply a more or less arbitrary label, like "Wolf" and "Dragon" for "Game of Thrones", which became somewhat well-known.

I tried googling it, but audio engineers apparently do use a measurement called a "volume unit", which makes the term so common in the general context of television production that I quickly gave up. There are audio credits under the heading in question, but nowhere near enough to make it seem likely that that has anything to do with it.

What did occur to me is that a counterpart to "volume" could be "surface", as in geometry, and in turn a counterpart to "surface" could be "aerial", as in footage. So it could be the unit responsible for the flight sequences, in which the viewpoint is much more likely to move and/or point vertically instead of only horizontally than in ground sequences.

Another sense I considered is the "in volume", like "in large amounts", one. Crowd scenes, say, need large numbers of extras, or their CG equivalents. Feels way implausible, though.

Thanks for any insights! :)

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ETA: Ah, I went through the 100+ occurrences of "unit" in the /fullcredits page linked above now, and there were a couple of other uncommon ones:

- Bailout unit

- Pick up unit

- Splinter unit

I'm guessing it forms a set with some or all of those. Still not really clear on what any of the terms refer to, though. "Bailout" is the one that's most directly linked to flight, to my mind... but is in colloquial use as well, of course, so... *shrug*

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ETA: Ah again, now I googled those other terms - and for one of them, that actually yielded a direct definition (howtofilmschool.com/film-television-dictionary-letter-s):

Splinter Unit: A part of the main filming unit that splinters off to get alternate shots.

The other two, just as the original one, are either too uncommon overall, or too common in unrelated contexts, but googling "splinter unit" alongside each of the others shows that none of them are original to "Masters of the Air", at any rate. So the first word does in each case have a particular meaning, even if I don't know what this is yet. Progress! :)