Is there a proper technical term for (physical) sections in a card file?
Solution 1:
Tab Section, Section, or just Tab, the last in the sense of contents of a tab section.
Use Case
Loletrazina Church, How to Use a Rolodex (bizfluent):
Arrange the cards alphabetically in your file. For example, place … the cards of people and businesses that start with the letter "A" in "A" section of the Rolodex.