My understanding is that all tobacco is cased at some point early in the production. This is done to make the leaf smokeable as apparently raw uncased tobacco can be very bitter. This is usually an application of sugar water or liquorice root extract in liquid form and is sprayed onto the raw leaf.
Topping is the adding of flavouring agents such as tonquin, whiskey, fruit essence etc to create aromatic blends. These are usually applied towards the end of the manufacturing process in large tumbling machines with spray nozzles.
That is how I see it, others may well disagree.
Regards,
Jay.