External files are associated with [B/D] by input and output channel numbers. Input channels link [B/D] to external files which contain [B/D] programs and/or subroutines. The links between channel numbers and external files are created by using the CHANNEL: command, and they may be removed by using the CLEARCH: command. The PATH: command may be used to allow the searching of other directories for any external files required. In cases where [B/D] is to share external files with other programs, such as text processors, the REFRESH: command should be used to update the external files.
Output channels offer destinations for output alternative to the screen, and are also used for the exportation of data and beliefs. Output is redirected by using the OCHANNEL: command, and data and beliefs may be exported using the ECHANNEL: command.