In my mind, backup contents should be irrelevant to current configuration path.
Next two examples should create two identical backups:

example 1:

1. set interfaces loopback lo description test1
2. commit

example 2:

1. edit interfaces loopback lo
2. set description test2
3. commit

Now, let's examine contents of the backups. Backup from "example 1" contains full config backup while backup from "example 2" contains only the configuration of the "lo" interface.