@ Matty
there are 2 scenarios in the COLREGS
a) restricted by the nature of their work, like a dredger
during dredging (not when not dredging like moving
from port to port, job-job etc, then she is a reg vessel)
also pipelayers like this for example, again only when
really doing pipelaying work

b) draft restricted
Which means of course a deep draft vessel which can not
use another lane or change courses outside a lane because
there is not enough water.

some other vessels are also restricted, for example, fishers when fishing, but for them is a extra section in
the COLREGS, same for Mine sweeping etc, but not Navy vessels in general, they are in the COLREGS for regular vessels. Some also abuse it , like a lot fishers who have still in the ports the fishing lights on (drifter have red over white and trawler green over white, all 360 degrees) A fisher is by defenition a vessel engaged in fishing and not a fishing vessel for example, when she moves without fishing she is considered as a regular power driven vessel