RAID Mirror with 2 Harddisk

Hi,
today i setup an Mirror with 2 HDD (2TB). After that i got 2 Volumes.
dev/md0 (2TB)
dev/md0p1 (800GB)

dev/md0 is fine (the whole Space of the hdd, 2TB)

But for what is the dev/md0p1 volume is used for?

Thanks