Illustrated by the output of dix_xmit -h below (Note the ". Default [...]":

Usage: ./dix_xmit [options]
IX Dolphin based transmitter

Options are as follows:
 -h, --help               Show this help.
 -b mbuf name             Input buffer name. Default [local_dc]
 -m [20-100]              Input buffer size in mb. Default [100]
 -g [0-3]                 The IX memory group to output to. Default [0]
 -p epics prefix          The prefix for epics PVs that report status
 -l filename              Log file name
 -v                       Verbose output