There are different ways to classify the types of companies that participate in the IESO-administered markets. One classification is based on the company’s physical assets. Some companies have their own equipment that produces or uses electricity; some companies participate in the market without actually producing or using electricity.

Companies with physical assets may be connected directly to the IESO-controlled grid, or they may be “embedded”. Embedded companies are connected to a distributor's lines, which are themselves connected to the IESO-controlled grid.

A market participant may play more than one role in the market. For example, a market participant could be both a generator and a wholesaler.