$4\mathrm{.}2-6.$ HOL blocking. What is meant by Head of the Line $($HOL$)$ blocking?

A queued datagram waiting for service at the front of a queue prevents other datagrams in queue from moving forward in the queue.

A queued datagram receiving service at the front of a queue prevents other datagrams in queue from receiving service.

In a block error code, the first bytes of the code indicate the type of coding being used.