The attached/linked document 9940TN02 is an application note with an overview of how to do low differential mode (negative gauge mode also) with a PG7601 (PG7607 and PG9600 also), and an uncertainty analysis which addresses the specifics of operating in differential mode which are not specifically addressed in the PG7000 Uncertainty Analysis.
This application of differential mode includes negative gauge mode. See Section 3.9.4 of the PG7000 Operation and Maintenance Manual for more information and instructions for using PG7601 or PG7607 in low differential (also negative gauge mode).
The attached/linked document "Expanding Use of the PG Webinar.pdf" is the presentation from a Webinar about this topic.
Note that RPM3 is no longer available and that the 3990 manual pressure controller can be used instead of the PK-7600-DIF differential mode controller.