If you need to copy a Document Definition from one FlexiCapture project to another, you can use the Import Document Definition functionality available in Project Setup Station > Project > Document Definitions.
Click Import, navigate to the source project's folder, open the Templates subfolder and choose the needed Document Definition's .fcdot file:
If you use the Distributive version of ABBYY FlexiCapture 12 and the source project was already uploaded to the server, you would need to export the project from the server first.
To do this, open the Project Setup Station and use Project > Export Project. This will export the project to the folder you specify and make it available as the import source.
Alternatively, the CLI for project administration (FCAdminTools.exe) that comes with the Distributive version of the ABBYY FlexiCapture 12 can be used. You can copy a Document Definition from one project to another by using the CopyDocumentDefinition command.