We don't use AC for billing but I just created a CMS 1500 form to have a look. Field 25 for us (v6.3.3) said "use practice ID." So I played around and went to edit user info in admin options. We have only 1 doctor and in her "edit existing user" tab, the TaxID box said "use practice ID." So I changed it to her EIN and the CMS 1500 did update to show the EIN in field 25.

Serene
Office Manager
General Pediatrics
Houston, Texas