There is no context name here!
There is an address of record and an authentication user. Most people do not have a distinct authentication user.
The address of record normally matches the section name, although there is a configuration option to make it match the phone’s authentication user.
Address of record is a concept for register. If you uses type=peer, which is best practice, in most cases, the section name is only used for register, otherwise it is matched against From, or is it contact, on invites. If you use static addresses, and type=peer, no matching is done at all.
The address of record should not be easily guessable (the common GUIs are very bad in this respect), and, as such, shouldn’t be a security issue.
If you use a separate authentication user, the sip.conf option to specify it should be fairly obvious from the sample configuration, as should be the option to match by that, rather than the address of record.