Inbox and Shared Files
Inbox is ideal to communicate directly with vendors. After selecting compose there is a drop down selector of those vendor using this paid feature. Those vendor with a free plan and are unable to communicate using this tool.
Vendors can also send in-app messages to partners. For instance they might advise of new features that can be promoted.
Shared Files is a paid feature that enables vendors to provide additional information to their partners. Many partner like to produce additional content to further inform their audience of the features of a particular product. This information (not available to the pubic) might include "cheat sheets, product briefs, logos and educational videos.