(Answered) How do 'Mech UMUs work? Includes an apparent BMM/TO rules disparity

[Alfaryn]

PP. 20 and 117 BMM don't say anything about a possibility of vertical movement with UMUs. They seem to treat UMUs as some sort of underwater jump jets. The rules for 'Mech UMUs on p. 295 TO coupled with the rules about Submarines on p. 56 TW seem to not only allow such movement, but even seem to require a Mech to be on a vertical depth higher then bottom to change facing or move horizontally using UMU (as is the case with the submarines).

Is this an intentional simplification in the BMM (which would probably require an errata to mark those rules as simplified, and probably an extra explanation about what it means that a 'Mech that used UMUs "counts as having jumped that turn" - does it only apply to to-hit rolls for attacks, required PSRs etc., or also to maximum elevation changes, cost to turn during this turn etc.) or an unintentional effect of rewording the rules?

Extra question regarding the rules in TW/TO. The rules for submarines say, that to move horizontally or change facing a submarine must not be on the bottom elevation. The rules for 'Mech UMU say that a 'Mech using UMUs moves like a submarine, but at the same time say that a 'Mech can not use UMUs to move at depth 1, as it is two levels tall and must be fully submerged to employ UMU movement. The problem with this is that if a 'Mech is standing in depth 2 water, and uses UMUs to move it would have to move one level up before doing anything else, but it can't do it as it would no longer be fully submerged. So should a 'Mech be in depth 3+ water (not 2+ as p. 295 TO says) to be able to use UMU movement or can 'Mech use UMU movement to move horizontally or change facing using UMU without going above the bottom level first (unlike a submarine)?

[Xotl]

The rules for moving vertically in the Manual were accidentally left out (I just noticed this two days ago and they will be added back in the next errata release).

However, a mech can move at depth 2: it is presumed to be hovering just above the water floor.  This will be clarified in both the BMM and TO.

[Alfaryn]

Thanks. On that note the Underwater Movement (Non-Naval Units) on pp. 56-57 TW rules should also clarify what it means for the UMU-equipped units to move "like a submarine". My guess that if 'Mechs can do it, then units such as UMU-equipped Battle Armor (TW/TM unit) or SCUBA Troopers (TO unit) can also use UMU MPs to move horizontally while at the bottom level.

Edit: This may require an errata to p. 57 TW  (under Underwater MPs) regarding UMU movement at the bottom level (not just TO/BMM), so that UMU movement of units other than 'Mechs is also covered. It should be also clarified if Underwater MPs allow units other than 'Mechs (like UMU equipped BA - fully covered in TW/TM, and SCUBA Troops - covered in p. 341 TO), can do everything else a submarine can do (like move vertically in water, or reaching the surface level, like submarines can per p. 56 TW, but 'UMU equipped 'Mechs can't per p. 295 TO).

Edit: I feel, that since rules for vertical UMU movement were left out, the UMU section on p. 20 BMM should be marked as simplified, and should clearly state, that unlike jumping the 'Mech still needs to pay MPs for turning and changing levels.

Edit 2: The crossed out text above is no longer relevant considering Xotl's edit to his answer.