Cinnamon
Also known as Cinnamomum verum, Cinnamomum cassia, cassia, Ceylon cinnamon, true cinnamon, canela, Zimt, cannelle
Two different barks sold under one name. Ceylon cinnamon is delicate and expensive; cassia is the hotter, cheaper bark that most jars actually contain, and the two are not interchangeable in a delicate formula.
Cinnamon in a shop is usually not the spice its name suggests. True or Ceylon cinnamon is the inner bark of Cinnamomum verum, pale, brittle, layered like a cigar and delicately sweet. Cassia is the bark of several related species, chiefly Cinnamomum cassia, and it is darker, thicker, harder and considerably more assertive, with a hot, almost peppery edge behind the sweetness. Most ground cinnamon sold in Europe and North America is cassia. Both are legitimately labelled cinnamon in many markets, so a recipe that simply says cinnamon is telling you less than it appears to.
In cured meats cinnamon is a southern and eastern spice. It runs through the Spanish and Portuguese blood sausages, the morcillas and the chouriços, where it sits with clove and pepper against the richness of blood and fat. It appears in the Middle Eastern and North African sausages, in some Greek and Balkan work, and in the sweet-spiced Chinese cured sausages. Northern European charcuterie uses it sparingly, mostly in brawns, potted meats and the Christmas-season products. It is essentially absent from the Italian dry salami tradition.
What it contributes is warmth with sweetness, from cinnamaldehyde, which makes up the bulk of the essential oil in both species. Cassia carries more of it, which is why it reads hotter. Cinnamon also brings genuine antioxidant activity, useful in a fatty product, though as with garlic this is a bonus rather than a hurdle you can count on.
There is one substantive difference between the two barks beyond flavour. Cassia contains coumarin, and Ceylon cinnamon contains very little: measurements put cassia in the region of 1,880 to 3,260 mg per kg against a highest recorded 185 mg per kg for Ceylon, roughly a hundredfold difference. EFSA has set a tolerable daily intake for coumarin of 0.1 mg per kg of body weight, which a 60 kg adult would reach at around 2 g of cassia a day. That is a real consideration for someone taking cinnamon by the spoonful; it is not a meaningful one at charcuterie doses, where half a gram per kilo of meat means a generous portion of sausage carries a few hundredths of a gram of spice. Know the distinction, choose the bark for flavour, and do not let the coumarin question drive a sausage formula.
Ground cinnamon of either species loses its top notes quickly and settles into a flat woodiness. Quills keep far better, and cassia quills in particular are hard enough that a spice grinder rather than a mill is needed.