I'm not sure that I can give you a definitive explanation, but I think I see a point there you might have missed.

IMO, this ought to have been written:
"customer- and partner-facing"

The point about a portal is that it offres facilities to BOTH potential 'customers' (i.e. surfers on the Net) AND 'partners' (the people whose sites you are presenting, and hence who want to sell.

So the position of someone running a portal site is like a pane of glass, with cutomers on one side and suppliers on the other; each of these needs to be able to look at the portal and interface with it in a way that is appropriate for their needs, and the portal acts as a go-between.

So it will be important that the software can address the needs of both groups; I am not clear whether the sentence means 'a piece of software [= application] that can address either customers or partners' (in fact, really both/and, I think), or if it means more simply the traditional meaning of 'application', i.e. 'can be used [= applied] for both customer and partners'

This is only my understanding of what it should mean as a lay person, I am no computer expert!