1) Reason to stop the presses 
Two or more different forms of an advertisement which are ran simultaneously in different copies of the same publication, used to test the effectiveness of one advertisement over another to appeal to regional or other specific markets.
Printing of two or more variants of a promotional ad run on an Nth or AB split throughout the entire edition. Use of geographical segments of a publication for testing of variants.
1. Different images, such as advertisements, printed in different editions of a publication.
noun a print run of a newspaper during which some articles or advertisements are changed to produce a different edition