Ermm...In California an El Camino is a "light duty truck," just like say a C2500. Costs more to register than a "passenger car," but unlike one of those, you can legally swap a late model Big Block into it, because the state considers any truck under 10,000 lb "light duty."
CARB considers 8501-14000 gvwr "medium duty"...
Cite to CARB:
Your confusion might come from the idea of a 2500 series truck. General Millenial made a subset series of 2500 trucks which weren't true c2500s. They called them '2500 light duty' trucks and they came with 6 lug suspensions instead of 8 lug, and a thinner lighter weight frame underneath.
Supposedly it was a niche product for urban cowboys that wanted a slightly heavier duty product than a 1500 but didn't need the full capability the medium duty lines of trucks offered.
The part of not sure of offhand is the overlap with the 1500HD product, it could be that one led to the creation of the other.