Professional version allows you to connect multiple users to the same database at the same time. That includes multiple sessions by the same user (for example if the same file is opened with two program instances). All users will get the full (not only a read-only) access to the file.
When one user changes the database, others will see the change immediately (after they have changed the folder, or done some changes in the database).
The program has built in record level locking, protecting that different users can overwrite others changes without knowing. If both users are changing the entry, then the first one who saves, will succeed. The other user, should he decide to save the changes, will get the error message saying that during the time they entered the change, the record has been modified already and thus the change cannot be made. Once the database is refreshed, and the second user will see the change that the first user did, but still wants to change the entry, he can then open the entry window again to make and save the change.