It is hard to find anything right now with the Help files so messed-up.
But,
The CMIS Properties tab will only display info if the file you are looking at has been opened from a remote connection to a content management system.
The information is read-only so you do not need to be concerned about entering anything there.
From Help (buried)
Properties of files stored in CMIS servers
Files stored in CMIS server have properties and metadata not available in a local storage. These metadata are important for controls and debugging of the CMIS connection and server implementation. All parameters displayed are read-only.
Choose File - Properties, CMIS tab.
If you are curious take a look at the Remote Services listed when you try to open a remote file.
File > Open Remote > Add Service button > Type drop-down list
If you have opened a file from one of those remote services then you may see something in the CMIS Properties tab.
If this answered your question please check the checkmark at left.
.