Start a Post (Discussion), Share a Case, and Referrals are post categories that help guide post creation and organization. All posts are tagged with whichever category you select when creating your posts.
Practically speaking, this is the only difference between the post categories. This may not seem important, but it has a real benefit when you read posts. When looking at posts in your feed, you can quickly and easily see the category of posts or filter by posts according to these categories.
Let's say you're interested in learning which colleagues are requesting referrals. Simply click the Filter by drop-down menu that appears above the first post in the feed, select Referrals, and only posts related to referrals will appear.