The delay pools are, essentially "bandwidth buckets." A response is delayed until some amount of bandwidth is available from an appropriate bucket. The buckets don't actually store bandwidth (e.g., 100 Kbit/s), but rather some amount of traffic (e.g., 384 KB). Squid adds some amount of traffic to the buckets each second. Cache clients take some amount of traffic out when they receive data from an upstream source (origin server or neighbor).
The size of a bucket determines how much burst bandwidth is available to a client. If a bucket starts out full, a client can take as much traffic as it needs until the bucket becomes empty. The client then receives traffic allotments at the fill rate.
The mapping between Squid clients and actual buckets is a bit complicated. Squid uses three different constructs to do it: access rules, delay pool classes, and types of buckets. First, Squid checks a client request against the delay_access list. If the request is a match, it points to a particular delay pool. Each delay pool has a class: 1, 2, or 3. The classes determine which types of buckets are in use. Squid has three types of buckets: aggregate, individual, and network:
A class 1 pool has a single aggregate bucket.
A class 2 pool has an aggregate bucket and 256 individual buckets.
A class 3 pool has an aggregate bucket, 256 network buckets, and 65,536 individual buckets.
As you can probably guess, the individual and network buckets correspond to IP address octets. In a class 2 pool, the individual bucket is determined by the last octet of the client's IPv4 address. In a class 3 pool, the network bucket is determined by the third octet, and the individual bucket by the third and fourth octets.
For the class 2 and 3 delay pools, you can disable buckets you don't want to use. For example, you can define a class 2 pool with only individual buckets by disabling the aggregate bucket.
When a request goes through a pool with more than one bucket type, it takes bandwidth from all buckets. For example, consider a class 3 pool with aggregate, network, and individual buckets. If the individual bucket has 20 KB, the network bucket 30 KB, but the aggregate bucket only 2 KB, the client receives only a 2-KB allotment. Even though some buckets have plenty of traffic, the client is limited by the bucket with the smallest amount.