FileMaker is increasingly part of an ecosystem where it needs to send to and receive information from third party services and applications. Often the most common exchange mechanism is via REST APIs. Without these, customers are unable to communicate with key third party systems to extend functionality or must resort to 3rd party tools (example: REST fm) to get the job done. These alternatives are costly, hard to maintain, require reliance on third parties, and are not FileMaker provided solutions. In addition, the XML and PHP APIs aren't best suited for the tasks REST API is meant for. Through a REST API we can provide a better alternative for those cases where Custom Web Publishing (via XML queries and PHP APIs) are being used for pure data exchange. Some customers expressed opinions that penetrating the IT barriers is easier if REST APIs are available.
FileMaker Data API is FileMaker's platform to integrate with 3rd party applications and web services and is part of FileMaker server family. For example, Tableau Integration is built on top of FileMaker Data API.
FileMaker offered a trial of the new the Data API feature in FileMaker Server 16. This allows users to create OBDC or SQL links between their own FileMaker applications and other programs for outbound / inbound data queries and transfers. Crate Pro isn't set up to use any API transfer so this feature would apply to FileMaker databases that you create independent of Crate Pro. With the release of FileMaker Server 17, there is a limit of 2GB per CAL per month for outbound data.
If you are using your FileMaker only for Crate Pro you can ignore the trial message.