Capacity is a type of commodity associated with gas and power, and gives a trading organization the option to "move" gas and power through the respective networks (pipelines and grids)

Detail

Capacity may be bought in short or long-term auctions directly from the Transmission System Operators (TSOs), or may be traded bilaterally

Ownership of capacity entitles the owner to transport gas or power from one part of a network (location) to another

A trading organization does not need to buy capacity to buy and sell a commodity at a location, it does if it wants to transport the commodity to a different location 

For example capacity on the Interconnector France-Angleterre (IFA) entitles the owner to transport power from the UK grid to the French grid or vice versa

As capacity may be used to change the location of a commodity, it is somewhat similar to an option on a (physically settled) location spread and is usually valued as such