Shared IP
A sending IP used by many customers of a provider.
Most senders start on a shared IP pool. Reputation is collective: one bad sender can drag the pool down, but established providers actively manage their pools. Below 100k/month, shared IPs are usually preferable to dedicated.
reading this as developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to shared ip, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.