Each horse should have ownership information recorded so that charges to the horse can be passed on to its owners. This information is entered to, and modified via the Owners tab in the horse information window.
Note that you will need to enter details for owners of horses in your care to Ardex before owners can be assigned to horses. The Sell Share button allows you to record changes in horse ownership over time, as owners sell shares in horses to one another. For more information see the Horse Ownership guide.