This product has an absorbent glass mat embedded in the structure of the plates. (There are 6 sets of plates, each 2.0V which makes up a “battery” of plates, i.e. 12V). The mat does two things. It reinforces the lead frame of each plate, or cell, and it stops the plates from buckling under heavy discharge current). By helping make the innards of the battery virtually solid, you can mount it in any position, but it is still preferable, of course, to mount the battery upright, even for the sake of convenience of terminal access. They are an excellent product. 

Note: It is important to not discharge an AGM more than 60% between recharge.