The Pro subscription includes all functionality of the system and the main feature that it has that the Add-On user does not have is the Sales Message Builder which is the area where you create your sales message and that is used to fill the library of scripts, emails, and templates.
The Add-On user is slimmed down and designed to be linked to a Pro user account and use the sales message that is created and saved in the Pro user account that it is linked to.
The two accounts are designed in a parent/child type of relationship with the Pro being the parent and the Add-On being the child. Rolling this out across a sales team will require one Pro account for either the manager or admin user. And then an Add-On user account for each salesperson that needs access to the system and all of those accounts will be linked to the Pro user (parent) account.