The trial version has no limitations. But if you would like to use HarePoint Workflow Manager for SharePoint after the 30 days evaluation period, purchase and registration is required.
There are 3 license types for HarePoint Workflow Manager Extensions for SharePoint 2016/2013:
1. Trial License. Provided for a 30-day period to use the product in the organization’s environment without any limitations of product functionality.
2. Free License. This license allows using the product with the Free Set of Activities without limitation of number of servers or usage time. After expiration of the Trial License, the product will continue to operate under the Free License by default. The set of available activities will be limited to the Free Set. Execution of workflows created by using other activities will be terminated with errors.
3. Full License. This license is purchased for a given number of SharePoint Servers and allows using the product in the organization’s environment without limitation on usage time or on available activities