Minecraft wiki entry for Development_versions, associated text: Made changes to match recent redesign:
Blooms when mobs die within an 8-block radius.
Mobs will not drop experiences when perished in the presence of the block, a bubbling charge will be created where they perished instead.
The charge from mobs will spread through Sculk Veins and Sculk blocks in random directions until they find a valid substrate that they can convert into sculk.
The value of the charge is directly proportional to the amount of experiences the perished mob would have dropped, and each time a block is converted into sculk, it will remove 1 value from that charge.
This charge in the sculk blocks and sculk veins will eventually decay, but it will decay much, much slower in the close vicinity of the sculk catalyst, and much faster away from its host.
If the charge decays 4 blocks away from the catalyst, it has a chance of growing a sculk sensor or a sculk shrieker.
Charges and their values also merge when they move to the same position.