links are also used to create a message thread against an object that was defined by a transaction id not by a signature as you can see below.

Apertus can create threads in several ways.

#Twitter like hash tags. (uses a translated public address with no known private key)
Etchings using a signature, vault or profile. (uses an address with an owned private key)
Using the link feature. (uses a series of backwards linking transaction id's)