ECP, short for Enhanced Contactless Polling, is Apple's extension to the way a contactless reader announces itself over NFC. A plain reader polls for any card in range and waits to see what answers. An ECP reader adds a description of what it is asking for, so an iPhone or Apple Watch in the field knows which merchant is calling and which kind of pass is wanted before any exchange begins.
That announcement is what makes auto-presentment possible. The device matches the polling frame against the passes in the wallet, selects the right one and has it ready as the phone reaches the reader, with no unlock, no wallet app and no scrolling by the member. Without it, the phone would have no way to know which of a dozen passes the reader wants, and the member would have to choose one first.
ECP is the layer underneath rather than a feature a merchant configures directly. It carries Apple VAS identification at the till, and the same mechanism is what lets transit and access readers surface the correct pass at a gate or a door. What a merchant actually configures is the pass and the reader: an NFC-enabled pass with the right key, and a reader or payment terminal set up with the matching key and merchant ID. The polling behavior comes with hardware that supports it.
Android does not use ECP. Google Smart Tap runs its own exchange to reach the same result, and a reader configured for both serves either phone from one tap target, so a program is never choosing between them.