HBD is backed by HIVE. The debt limit prevents us from printing too much HBD, which would lead to a LUNA like death spiral.
I actually remembered older numbers, the last hard fork changed the percentages.
When the total amount of HBD in circulation exceeds 20% of HIVE market cap, the blockchain will stop printing HBD for rewards. At 30% and above, it won't honor the $1 peg with conversions any more.