Users on Hive have the ability to select what form they want to receive payment from blog posts.
The 50/50 payout options means the users is willing to receive 50% of the payout in HBD with the other 50% coming from Hive Power (HP). This is in contrast to the 100% payout in HP.
When this option is selected, if the haircut rule was kicked in, the HBD will be substituted with liquid $HIVE. This is a security feature of the blockchain that protects against the debt of the ecosystem in relation to the value of the main coin from getting to high.
Payments will cease to have HBD included until the ratio falls below the level coded at the base layer.
As of Hard Fork 26, this was raised to 30% from the previous 10% level.