Optimistic concurrency has been enabled for estimates, sales orders, sales receipts, and invoices. This means that two people can edit the same transaction concurrently. The first user saving the transaction will not see any difference during the save. However, the second user will see a pop-up window with the notification, "A conflict occurred while saving," such as this one for a sales order:
The second user has the option to either discard their own changes or overwrite the changes made by the other user.