What is a discussion?
When you use the "Start a Discussion" option, it posts what you want to say on the feed and posts it in the "Discussions" area. This is what you would use if you just wanted to post text with no picture attached to it.
Anything you want to just 'say' to everyone in the group, should be posted under "Start a Discussion".
You can find the Discussions on the Discussions tab on the web, or by tapping Discussions along the top navigation on the mobile apps.