Technically, the SBD "disappear" from the system. Whether you want to call this "burning" or "booking out" is probably a matter of preference.
All transferred SBD are set back to 0 with the next block. In addition, current_sbd_supply is reduced by the amount and virtual_supply is reduced accordingly.
In the current phase, this would be particularly important, as it would then reduce the debt ratio again.
However, for this to have a noticeable effect, quite a lot of SBD would have to be "burnt"...
If the price rises as a result, this has an additional positive effect on the debt ratio.