The three main models
Market maker (B-book): The broker takes the opposite side of every trade. Creates a conflict of interest but typically offers fixed spreads.
STP (Straight-Through Processing): Orders are passed to external liquidity providers without a dealing desk. Variable spreads, usually tighter than market maker.
ECN (Electronic Communications Network): Orders are matched directly against other market participants. True raw spreads, transparent commission per lot. Best for scalping EAs.
For EA deployment
Market-maker brokers may restrict scalping EAs or impose minimum order durations. ECN/STP brokers are generally EA-friendly. Always verify the broker EA policy before depositing.