What is the purpose of slack space in vSAN?

What is the purpose of slack space in vSAN?

A. Increases overall vSAN capacity

B. Allows for maintenance and failure operations

C. Stores vSphere HA heartbeats

D. Provides vSAN analytic data

Reference: https://storagehub.vmware.com/t/vmware-vsan/vmware-r-vsan-tm-design-and-sizing-guide-2/how-much-slack-space-should-i-leave-1/