This functionality allows you to keep information on suppliers or external collaborators in a comprehensive way - Parties Overview.
This overview can be found by opening "My Name"->Overview->Parties
Click the "Third Parties" tab to add a new third party or filter third parties in the full-text search
You can also add a third page in the file tab by clicking on the "gear icon".
Then, on the file information tab, you can select a third party from the list, or you can add a new one.