Shared IP Pools
IP addresses are the actual "locations" from which the emails are sent. Elastic Email operates and sends email from thousands of IP addresses. Many of these IP addresses are grouped together into what is referred to as an "IP Pool".
There are several tiers of IP pools, and the shared IP pools are ranked within each tier. Depending on certain metrics, your account will automatically be moved in and out of IP Pools.
Why does Elastic Email do this? For a lot of reasons. And it can get complicated. But, generally, it's all about optimizing email delivery automatically for a wide variety of senders.
Each account has its own "Reputation". Basically, it's like a health scorecard for the account. Details about how this score is calculated can be found here.
Your account's reputation is one of the factors that determines which IP Pool your account uses. There are a few other factors, but maintaining a good reputation is essential.