What it means on the back of your 404s is that you do not need to produce a civilian driver's license to the civilian police in the event of an incident. When driving SMP or DND vehicles, your 404s are your provincial driver's license.
You are required to have at least a G1 (in Ontario) or equivalent to drive DND vehicles. Once you have that, you can be issued a learner's permit or full DND 404s which will expire when your civilian license expires. If at any time your civilian license expires/gets suspended/gets removed you need to report it to your transport section and they will take appropriate action.
I learned to drive using LSVWs, then I took Young Driver's civvie side, having already 404s to drive LSVW with Trailer. All I needed was a G1, which I got 2 weeks before starting the LSVW driver wheeled course.