It was a fairly advanced protocol for its era, supporting efficient transfers of files, commands and other data as well, and could be used in both directions at the same time in certain modes.
Notable exceptions were Tera Term and Datastorm's ProComm Plus on the PC which featured the ability to listen for the Enquire command on the active communications port, and ZTerm on the Mac which allowed auto-starting transfers.
This development was part of a wider trend of using external communications applications in conjunction with online services.
This protocol was intended for use by a custom online terminal built by Tandy, the "AgVision" or "VideoTex", but this project was abandoned after being sold for only a brief period.
B Plus focussed the overall concept primarily on supporting downloads from CompuServe, as opposed to user-to-user transfers.
All potential problematic control characters were always quoted, a requirement because many people accessed CompuServe over non-8-bit-clean packet services such as Tymnet.
These packets were used both for data transfer as well as secure delivery of commands and protocol setup information.
This packet contained a number of details in a known format that synchronized what features both ends of the connection were capable of using.
Possibly the only user of the Transport Layer was CompuServe's own Host-Micro Interface (HMI) API.
Since error correction was being used as a side effect of being built on B Plus, the possibility of incorrectly interpreting the commands or potentially garbled responses was basically eliminated.
CIS expanded HMI to allow control of most of the batch-oriented interface, including functions for e-mail, conferences and file transfers.
For instance, CIS Navigator for the Mac, which was HMI based, allowed users to navigate CIS offline, setting up various e-mail and file transfers which would then be carried out in a single batch in order to reduce online time.
These included the typical DLE followed by a sequence number in order to acknowledge the correct reception of a packet.