Shared Boards are boards (a set of lists) that multiple people in a Team can view and edit. Shared boards have no predefined list names or any special functionality over and above a normal board other than that tasks on a Shared boards can be assigned to different people within the team. Each user of Sortd gets their own Personal Board which can not be shared, but additional boards created can be shared if all people who wish to share the board are on the Sortd Team subscription package. Consequently shared boards are not available on the free version of Sortd nor are they available in the Pro subscription which is a single user subscription (which does allow for creating additional boards over and above the Personal Board).
The shared boards only become shared when you invite someone to the board (who must also be part of the team), so they can remain private if you so wish.
At the moment only Tasks and their associated notes, dates, and assignments are shared, email on a shared board is only available to the person who dragged the email onto the board, and hence is not visible to other members of the team. Shared email on tasks is something that is planned for the future, but has complex security and privacy issues which make this a non-trivial issue.
For Pricing and Subscription information, see the Sortd pricing page at: https://www.sortd.com/pricing