Vehicles are required to have side markers that are visible directly from the side (same with reflectors). Most modern cars incorporate them with the headlamps and put them on the corner of the vehicle where they can act as both side and front marker lamps. They should be on anytime the headlamps are, and are allowed to be used as signals as well depending on local laws. Typically the wording is "located as close the the corner of the vehicle as viable" or similar. Basically, they want to be able to see where the corners of the car are at night.
Front side-markers are tough on a locost. In my case, front signals/markers are barely visible from the side over the fender (by design) combined with "fender" lamps located in my cowl. Signals and markers are visible from any angle so I figure they are serving the purpose.
Keep in mind they will be looking for DOT lamps during inspection to get it legal. If the exterior lamp is not DOT, and have the right markings for the location, it is not legal. This is one area that most places are pretty picky about. I used Grote MicroNova lamps for my side markers if it helps:
http://www.grote.com/family/micronova-d ... rker-lamp/Cheers.