In RAID level 6, two different parity calculations are carried out and stored in separate blocks on different disks.
The advantage of RAID 6 is that it has got a high data availability, because the data can be regenerated even if two disk containing user data fails. It is possible due to the use of Reed-Solomon code for parity calculations.
In RAID 6, there is a write penalty, because each write affects two parity blocks.