Lamb Shoulder is the traditional name in United States. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Shoulder is the meatier substitute when a recipe calls for breast, with the same fat-to-collagen behaviour over a long cook.
Shoulder is the standard swap for neck in lamb stews and tagines — more meat per kilo, same slow-cook requirement.
Same age-class distinction — cordero lechal is roasted whole or in quarters, regular lamb shoulder is butchered as a primal cut.