Both editions share the same ISO. The edition is unlocked by the product key you enter during installation.
Elias’s heart hammered against his ribs. He reached for the ethernet cable to pull the plug on the isolated test machine, but the text continued. --- Microsoft Exchange Server 2016 Iso Download
Unlike older versions (2010/2013), Exchange 2016 does not have separate Client Access Server (CAS) roles. The ISO installs the unified Mailbox role by default. Both editions share the same ISO