Simultaneous playback is the number of streams or live sessions that one account can run at the same time. In streaming services, this is an entitlement rule: it defines how many devices may watch concurrently under one subscription. The limit matters because it helps providers control account sharing, reduce abuse, and match service cost to usage.
From a security perspective, simultaneous playback is part of access control. Attackers who steal credentials may try to open multiple streams at once, share accounts across buyers, or keep a session active after the owner changes the password. Defenders monitor concurrent sessions, device identifiers, IP changes, and token reuse to spot abuse. If the limit is too strict, legitimate users may be blocked; if it is too loose, fraud becomes easier. So this setting is both a product feature and a defensive control that shapes how trustworthy and enforceable an account model is.