_Impl Source Files
In RODL-based server projects, _Impl files (short for "(Service) Implementation") will be generated by the IDE for each service you define in the .RODL. Once created, the file becomes "yours", and you will implement your service logic in it. The IDE will not touch or re-generate it.
As you migrate your server to Code-First (see the Converting Your Server to Code-First topic for details), your _Impl files remain intact, but you can then choose to drop the same suffix, if you like.