This read-write value specifies the byte pattern the channel transmits toward the line, filling in for missing payload, whenever the channel experiences a buffer underrun, applicable only to CESM-8T1/E1. An administrator sets this to a specific conditioning pattern expected by downstream equipment so an underrun produces predictable, benign output rather than random noise on the line. For example, setting a standard silence/idle code here ensures a brief underrun on a voice channel produces silence rather than an audible glitch.