Are you talking about a typical split system involving an air handler and/or furnace and condensing unit? See image below to make sure we're talking about the same types of equipment. Is this what you are thinking of? If so, slanting is not really an option because it needs to properly drain the condensate, and a slant would likely cause the drain pain to overflow and not drain properly.

Here is a link that might be useful:

I think that he/she means a minisplit unit not a split system..that is my take on it!

with a mini split the condensing unit goes outiside
just like with a split system. the ahu (air handling unit)
hangs on an exterior wall..where as with the ahu of a split system goes in a heating closet, basement or attic.

the mini split unit will have not only a drain or condensate that will need to be set up to drain water outside but also the refrigerant lines attach to both
ahu and condensing unit.

I've never seen one mounted other than horizontally,
and never on any but an exterior wall. Others may have
different experiences, these are just mine.

best of luck.