Can someone define Heavy-duty lampholder for me. Thanks.

I believe this is going to be a listing and manufacturer specification. The code requires heavy-duty lampholders when connected to circuits greater than 20-amp. A heavy-duty lampholder will not have a rating less than 660-watts if of the admedium type and not less than 750-watts for any other type. Basically, these are you HID luminaires. 210.21(A)