How ECN differs from market maker
On an ECN, your buy order is matched against another participant sell order at the best available price from the pool of liquidity providers. There is no broker dealing desk. Spreads can be very tight (0.0-0.3 pips on EUR/USD) with a transparent commission per lot (typically $3-7 per standard lot per side).
When ECN matters for EAs
Scalping EAs require ECN or STP execution. A market maker can widen spreads at will during fast markets and may restrict scalping by policy.
Swing EAs trading H4/D1 are far less sensitive to execution model — the difference between ECN and STP is irrelevant when holding trades for days.
True ECN vs ECN marketing
Many brokers market themselves as ECN while operating a hybrid B-book model. Verify by checking whether the broker charges per-lot commission (a strong indicator of genuine ECN/STP) rather than relying solely on wider spreads.