Tickets Available = the amount of tickets available for a particular ticket type.
Event Capacity = the maximum amount of tickets available for your entire event.
Sticky Tickets is designed so you never exceed your event capacity. So for example, if the maximum amount of tickets you can sell is 200 (event capacity) and you have three different ticket types (Adult, Child and Concession) but you don’t mind how many you sell of each type – then the tickets available for each type can be set to 200. Sticky Tickets will never sell any combination of tickets past the event capacity of 200.