One part of this statement isn't exactly correct. The TEU is used to measure the capacity of container ships as well as the cargo volume at a container terminal or port (i.e., "The Port of Savannah handled [X] TEUs in 2015"), but not the capacity of a container terminal. The capacity of a container terminal is dictated by a combination of factors, including ship loading/unloading rates, storage capacity on the pier and nearby container storage yards, gate capacity at the terminal gates, and the speed at which containers are picked up and dropped off by customers. Capacity is usually posted in terms of container lifts (one container lifted on or off a ship), and is measured in terms of lifts per acre for a unit of time. For example, the capacity of Terminal [X] at the Port of Long Beach might be measured as: "4,500 lifts per acre per year."
Thank you for the clarification.
I work at a marine/offshore class society and I write/edit class rules for vessels all day long - I should know this stuff!