When creating tickets in Event - Tickets you have the option to specify ticket limits. These limits ensure that no sales of any ticket type can exceed the amounts specified there. If no limit has been set, the number entered at Total number of tickets will be used as the limit.
There are 100 tickets available. The limit of the Regular tickets is set to 80, which means that a maximum of 80 Regular tickets can be sold. No limits have been set for Subscribers only tickets, Group tickets and Locked tickets. This means that it is possible to sell, for example, 100 Locked tickets, provided that they are ticked for sale. After all, there are 100 tickets available and no limit has been set for Locked tickets.
A limit has been set on Guest tickets; a maximum of 20 guest tickets can be provided. When only Regular tickets and Guest tickets are checked for sale (you do this by ticking Door or Web), then a maximum of 80 Regular tickets can be sold and 20 automatically remain for guests.