There is no catch with the free plan. You can freely use the full EPM product in your production environment on up to 25 workstations and 10 servers in your organization. Further, you can remotely access up to 25 workstations using the Secure Remote Access product.
You can have a Free Plan and/or a Paid Plan, but not a combination of both. This means that, if you want 35 licenses, you don't get the first 25 licenses for free and only pay for the remaining 10 - if you have a Paid Plan, you must pay for all licenses under that plan. However, you can have a Free Plan as well as a Paid Plan. This can be useful for testing purposes and is a common setup for customers.
For more information, refer to Licensing.