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Which Merlin is the right one for me?
That's a good question. It depends on what you want to do with Merlin and on your hardware.
The full featured application for project management on the mac is Merlin Project. This is the app you should buy if you require the pro features.
Merlin Project Express
If you are a mac user who does not need the full richness of Merlin Project, you can use Merlin Project Express.
Merlin Project Go
If you would like to manage your projects with Merlin on your iOS device, you can use Merlin Project Go. It can create new files, open existing files created by the Merlin Project, Merlin Project Express or even a third party application. Merlin Project Go cannot print, export, or do reports.
If you need to be able to collaborate with others (multiuser mode) this is possible for Merlin Project and Merlin Project Go.
Whenever Merlin Project and Merlin Project Go edit the same file over the iCloud drive or a Dropbox account, the applications understand the magic and show merged changes from any device as they come. If this is all you need, than you don't require a server. You may use iCloud drive, or Dropbox accounts and get updated files in the intervals those services do update their content.
Merlin Server allows multiuser mode for Merlin Project and Merlin Project Go. We call what the server does: syncing. Merlin Server syncing happens real fast as Merlin Project and Merlin Project Go communicate directly with the Merlin Server and do not depend on the cloud service.
Merlin Server allows in addition, that you use it for automated exports, and/or as a web server to offer your clients live views of your projects.