As of the Spring '21 Salesforce update, guest users are not permitted to create or update agreements.
When a sender has to prefill a field, that sender has to authenticate to Acrobat Sign to gain access to the field. This happens seamlessly through the integration, so users typically won't be aware of it.
If the agreement is also configured to Host the signing for the first signer, a conflict arises as the established session is authenticated to the sender, and not the first signer.
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.)
Open the Agreement Template > Add recipient related list > New button
In the Source of Recipient list, select "All Account Contact Roles/All Opportunity Contact roles".
It displays the Uniform Index check box.
Select the Uniform Index check box.
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.