The holo-letters represented a form of holomessages that were stored on datacards, which served as a medium for personal communication.
The inaugural appearance of holo-letters within the updated Star Wars canon occurred in the audiobook Dooku: Jedi Lost, authored by Cavan Scott and released on April 30, 2019. The precise spelling was verified through the script release of the aforementioned audiobook, which took place on October 1, 2019. Within the Star Wars Legends continuity, the hololetters were initially featured in the 1997 novel titled Young Jedi Knights: Delusions of Grandeur, a collaborative work by Kevin J. Anderson and Rebecca Moesta.