From the American Mathematical Society:
MathSciNet now includes Preliminary Data items, which are created from bibliographic data received directly from publishers. This data allows users to view early information about new papers in mathematics while full processing, including author identification and editorial decisions, is ongoing.
Preliminary Data items are marked as such until processing is complete. The initial group of publishers who have agreed to provide preliminary data feeds includes the American Mathematical Society, the Canadian Mathematical Society, the European Mathematical Society, the Mathematical Association of America, SIAM, Elsevier, and Springer. Publishers interested in participating in the Preliminary Data program are invited to contact: firstname.lastname@example.org.
In addition, new MathSciNet item tags allow users to instantly assess at what stage an item is in the review process. The seven tags are: Preliminary Data, Pending, Reviewed, Expansion, DML, Indexed, and Thesis. For more information and detailed descriptions of each item tag, please visit: www.ams.org/mathscinet/help/fullitem_help_full.html.