When [Shared BETA] is selected, your listing will appear in the New Search BETA search results in accordance with the Phase 3 New Sharing Rules.
There are individual [Shared BETA] tickboxes under:
For Sale
For Rent - Long Term
For Rent - Short Term
This allows you to independently share the property for sale and not share for rent, or vice versa.
Related articles:
What is the difference between Search 1.0 and Search 2.0 Beta?
What is the difference between the Shared and Shared BETA tickboxes?
Search 2.0 Beta - results page explained
Q&A - New Search 2.0 Beta