Document - add field for input or output documents

Hello,
When dealing with client projects, very often we have some documents which we require as input, and others which we write as output.
Would it be possible to add a field to make this distinction ?
For the moment, we've used types for this, but it would be clearer to have a binary distinction. Also, it is often the same types of documents we have as both input and output (e.g. some requirements documents as input, and also requirements docs as output).

Cheers!