The JewishGen collection search page has a banner at the top which describes the special collection:
See the Ancestry blog's post, New Jewish Family History Collection on Ancestry.com (Posted by Ancestry Team on October 29, 2008 in Collections, News), and Ancestry.com and JewishGen to Provide Online Access to Millions of Jewish Historical Documents posted on August 20, 2008 by Juliana Szucs on 24-7 Family History Circle for the initial press release.
At the bottom of this page, there is a box with links for all Jewish collections on the site.
Looking at the list, it seems that the included are either specifically related to Jewish history, or contain records for geographic areas where Jewish communities are located. Support articles such as Finding Jewish Records focus on explaining how to use the resources. I was not able to find any kind of "white paper" describing the selection criteria for the special collection.
Looking at Ancestry's About the Database section on the search page for the individual databases will give more information about the records in the database, who holds the original records, and why they were created.
For more information on the organizations which have partnered with Ancestry for this collection, see:
For background information, see the Ancestry Wiki article Overview of Jewish American Research, which originally appeared in "Jewish American Research" by Gary Mokotoff in The Source: A Guidebook to American Genealogy, and the related articles in the series (see the sidebar).