Bounds keep coming up in things but arent necesarily explicitly described in the text with exception of one phrase
From Text
“Moreover the upper bound in equation 15 depends on the storage costs, but not the convenience yield.”
It seems to me like a rule of thumb is, if you subtract it (i.e convenience yield, lease rate) it establishes a lower bound for the no arbitrage price range, if you add it (storage), it establishes an upper bound for the no arb price range.