The Uniform Index basically marks "All" the contacts in the object at the same recipient order, it means, it sets all the contacts to be able to sign in parallel.

Pre-requisites: It is exposed on the Agreement template, so it works with the Agreement templates with the Master object as Account/Opportunity. (An object that has a Contact-related list.)

While sending the agreement using this template from the Master object, all the contacts as the recipient with the same index are specified in the Index box on the Agreement Template.