regarding licenses, as per https://www.microfocus.com/media/documentation/additional-license-authorization-for-collaboration-solutions-products-documentation.pdf you probably should get inactive licenses for your use case.

The URL as shared by Marko has one thing you need to not overlook and thats "explicitly setting the mailbox to an Inactive License" which means in gwadmin on all the users you need to check this checkbox "Force Inactive status". Only then after 60 days or more the mailbox will fall under the Inactive license.

The audit report with gwcheck will report this as well like this:

...............................
Audit report for user (gwinactive) on database /gwise/mainpo/ofuser/userac1.db
Last login time - 05/21/24 11:01
Last activity time - 05/21/24 11:01
Forced inactive time - 05/21/24 11:23
User reported as inactive

User has been inactive for 114 days

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For Retain you need the same amount of "inactive licenses" as users that need to access the Retain archive.