I recently asked my wife (Chinese/Vietnamese) if we had any cassia bark at the hotel, as I had run out. She said "no, but we have some cinnamon". I was surprised, but said "OK, can I have some of that then ?". Needless to say, when it arrived home it was cassia bark (the coarsest kind) as predicted. When I explained the difference to her, she said "we have only the one word in Vietnamese, which refers to both. We [the Vietnamese] believe that cinnamon is thin young growth while cassia bark is older, tougher and more mature [but both from the same tree]".
Incidentally, Dr J S Pruthi, in his authoritative Spices and Condiments, devotes many pages to cassia and cinnamon, differentiating between Cassia (Jangli dalchini), Cassia China, Cassia Vera, Cinnamon (Darchini), Cinnamomum aromaticum, C. burmannii, C. cassia, C. inners, C. laurerii, C. obtusifolium, C. tamala and C. zeylaminum. The topic is clearly more complex than some would have us believe.
** Phil.