I keep hearing about cased tobacco. What...

... does this mean?
There are two things of interest here, namely "casing" and "top flavouring." They are two distinctly different approaches to altering a blend‘s flavor. Some tobaccos employ both.
Casing requires that the tobacco be soaked, literally, in a "sauce" that may contain sugar, molasses, liquorice, alcohols like rum or whiskey, and various flavourings, natural or otherwise, depending on the manufacturer. Once the tobacco "drinks" the sauce, it‘s conditioned in large cylinders that dry it back to the desired moisture level, generally between 12% (on the dry side) and 22 (very moist). Optimal moisture for smoking depends on the smoker, but it‘s generally in the 13-16 range. The aromas and flavours imparted by casing will remain in the tobacco pretty tenaciously, and will affect the smoke throughout the bowl.
Top-flavouring is added by spraying the finished blend with scents and flavourings. This is a much lighter application, and doesn‘t alter the moisture content of the leaf dramatically. Sometimes called "top-notes," this can be quite ephemeral. Because of the volatile nature of many of the commonly used components, a tobacco left to "air out" may lose a lot of the perfume that‘s applied this way.
Depending on the casing used, tobaccos can become very sticky. Some producers use humectants to maintain a specific moisture level in the final product. You‘ll hear people talk about PG, or propylene glycol, the most commonly used humectant these days. It‘s generally spoken of in rather disparaging terms, thought it‘s not the PG that deserves the condmenation, but the blending houses who use it with reckless abandon. If the tobacco won‘t dry out, PG is likely the culprit. In small quantities, it does its job well. In large quantities, it produces a sticky, wet smoking, pipe clogging weed that should never see the inside of a pipe.
Not all flavoured tobaccos are cased, and casing is not always a bad thing, but the term is used incorrectly more often than not, so a lot of confusion has been created.

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