This battery is "maintenace free", not a gel cel or AGM type: it's a modified wet cell battery and comes with a container of liquid battery acid you pour in to activate it (just like a standard dry charged lead acid battery). You can't put it on its side.
A gel cel can be put in any orientation I think, but I've noticed that every device I've ever seen that uses them orients them the same way, with the "top" up. Maybe they aren't super happy sideways or upside down?
The newer AGM batteries should be able to mount in any orientation as well from what I understand but again I've never seen an original equipment application where the battery is not mounted with the "top" up. This mounting may just be a habit left over from wet cell designing of course!
