The IBM 3270 class of terminals typically have traffic volumes greater than TTY access. Echoplex is not needed. The output devices usually handle higher speed output streams and most sites do not have the ability to stream input. Input is typically a screen at a time, but some PC implementations of 3270 use a variation of the protocol to effectively stream in volumes of data. Low delay for low volume input and output is appropriate.
High throughput is appropriate for the higher volume traffic.
Applications that transfer high volumes of data are typically streaming in one direction only, with acks for the data, on the return path. The data transfer should be high throughput and the acks should probably be regular class of service. Transfer initiation and termination might be served best with low delay class of service. Requests to, and responses from a time service might use low delay class of service effectively. These suggestions for class of service usage implies that the application sets the service based on the knowledge it has during the session.