It allows determining which documents will be associated to the task.
Pressing this 
button the following dialog will appear:
Then press again this button and a dialog that shows all the documents defined on the Knowledge Base will appear (more info).Then you must select the documents and the actions associated to them, these actions could be: read, create a new document, update or delete.
The Required Action column allows establishing whether a specific action to be performed on the document is mandatory or not to be able to complete the task. That is to say, it will not be possible to complete a task if it has mandatory actions to be performed on documents and these actions have not been complied to.
Note: this property may be used if the property Work With Documents has loaded the value True.
Objects: Business Process Diagram Object in Evolution (User Task Properties)
HowTo: Work with documents in GXflow
Work with documents property