If you use a branch or feeder from one building to supply power to another building, you generally can run only one branch or feeder for that purpose [225.30]. There are five conditions under which you can exceed this limit [225.30(A) - (E)]. The first one is "special conditions."

You can install additional feeders or branch circuits for:

1) Fire pumps (Art. 695)
2) Emergency systems (Art. 700)
3) Legally required standby systems (Art. 701)
4) Optional standby systems (Art. 702)
5) Parallel power production systems (Art. 705)
6) Systems designed for connection to multiple sources of supply for the purpose of enhanced reliability (Art. 705)

In the above list, each of the first four items is followed by its respective NEC article. Read each article, and you see what these are. It's clear how, for example, a legally required standby system differs from an optional one and how those differ from an emergency system and a fire pump system.

The last two items are both covered by Art. 705. The distinction between them is really one of quantity (see the definition of hybrid sources in 705.2). If you have additional power sources, you can exceed the one feeder/branch limit. You might have an outbuilding that's on solar power with the utility as backup.