The Moroccan term citron beldi (or l'hamd beldi) translates to "traditional lemon." It refers to specific varieties of lemon found in Morocco and favored in traditional cooking, including the small doqq lemons (Citrus limonum Risso var. pusilla R), which grow predominantly in the Taroudant region outside of Agadir, and boussera lemons (Citrus limetta Risso), which may also be called limonette de Marrakesh.
The latter is a type of Mediterranean sweet lemon which, according to Paris-based cookbook author David Lebovitz, may mistakenly be sold as "bergamot" in France. Limonette de Marrakesh and bergamot, however, are different fruits, as a true bergamot (Citrus bergamia Risso) is larger in size, much less palatable, and is appreciated mostly for its aromatic essential oil. Nonetheless, "bergamot" appears to be a term associated with the Moroccan lemon.
Both types of citron beldi are distinguished by their orange-yellow color at maturity, fragrant skin and unique shape – they're smaller and rounder than some other cultivated lemon varieties, and the bouserra has a flat apex and especially prominent nipple. While citrons beldis have general culinary value, they're most notably used to make l'hamd marakad and mssiyar, or Moroccan preserved lemons. The distinctively petite doqq lemon, as noted by Moroccan cuisine expert Paula Wolfert in her book The Food of Morocco, is considered the crème de la crème for this purpose, due to its thinner skin and fragrant aroma.
If you want to make your own preserved lemons and can't find a sweet Mediterranean or Moroccan variety, then try using Eureka, Meyer or other lemon variety of your choosing.