I've been looking at disk array calculators like the one shown here:

<https://www.synology.com/en-us/support/RAID_calculator>

I put in 16TB (4+6+6) and it says I can have 10TB of data and 6TB of protection. How can it afford to offer this? I would think that after striping 4TB it should be unable to protect the remaining 2TB, because there is not enough information to resolve parity

I assume the parity is compressed in some way. Is there a way to prove that a maximum quantity of space will be required for the compressed parity, or is it lying and would pull some hijinx like pretending the drive is full if it couldn't get the compression ratio it wants?