Printing: This is where you pre print your own tickets for your event. This is if you plan on selling your tickets at another location, through another person, or just want to have tickets ready to go in your box office.
Admitting: If you admit you are telling the system that this person has entered the event. This is usually when the box office acts as the ticket seller and gate keeper. No further action is needed for the customer to gain entrance to your event after you admit them.
Issuing: When you issue a ticket you are selling the ticket to a customer and they receive a printed ticket from your terminal in the box office. Issuing tickets typically used when the box office is separate from the main entrance and your customers need some sort of credential to enter the event.