Preempt_keep:

preempt_keep: < digis>

VariableComment
digis < keep>[,< keep>]... list of digipeaters to keep when preemption takes place on the frame, supports wildcards ?, #, @ and *
keep digipeater call to retain in the "via" list when the frame is preempted

example:

preempt_keep: PA*,PE*,PD*,PI*