Can you tell me where in the white paper? I don't remember reading about the ability to retire/destroy SBD.
I don't see a particular sentence but the concept is woven throughout the SBD section, for example in describing SBD as analogous to convertible notes, mentioning that conversion can only be done in one direction, etc.
Then how do we know which VESTS to transfer to the party who owns the SBD when a SBD is retired? Or what does it mean to destroy a SBD?
First of all converting SBD does not deliver VESTS, it delivers liquid STEEM (which can then be converted to VESTS via a power up). Second of all, VESTS are fungible so there is no such thing as "which VESTS".