ISP proxies are IP addresses registered to a consumer internet provider but hosted on datacenter servers. The one thing a buyer must know is that they are fast and stable like datacenter IPs while looking like residential ones, and they keep the same address for as long as you hold them.
The trick is in the registration. A website that looks up the network behind an ISP proxy sees a consumer ISP, not a hosting company, even though the machine answering the request lives in a datacenter. That is why they are also sold as static residential proxies, or residential ISP proxies. You can check any address you receive with a public IP lookup tool and confirm the network name and country before you rely on it.
They are usually sold per IP rather than per GB, sometimes with unlimited traffic. That makes them a fit for work that breaks when an IP changes, such as managing the accounts you are authorised to manage, long sessions and consistent geo checks. They cost more per IP than shared datacenter addresses, and most of the pool sits in a limited number of countries.