When the same character is active in two separate places at the same time. Cross-posting is not permitted in Nepta, and is considered bad form in many role-playing communities.
To post a single message simultaneously to multiple newsgroups or discussion groups. Gratuitous cross-posting is considered poor Netiquette.
Posting a single copy of a Usenet article to several newsgroups at the same time by putting their names in the Newsgroups line. More efficient than posting multiple individual copies.