DTR stands for Data Terminal Ready, it is a control lead presented
by the DTE (Data Terminal Equipment). The Verilink IDCSU is DCE
(Data Communications Equipment) which expects to receive DTR as an
input from the DTE indicating it is present, powered up and
prepared to operate.
If the DTE does not assert DTR this option allows the IDCSU
to behave as it would if the DTE were asserting DTR.