What does df command use to get "Used" column?

[Peter Steele]

A typical df command looks like this: 

# df -h 
Filesystem Size Used Avail Capacity Mounted on 
/dev/mirror/gm0a 4.8G 2.0G 2.4G 46% / 
devfs 1.0K 1.0K 0B 100% /dev 
linprocfs 4.0K 4.0K 0B 100% /proc 
/dev/mirror/gm0d 3.9G 88K 3.6G 0% /tmp 
/dev/mirror/gm0e 15G 79M 13G 1% /var 
/dev/ad4s3e 116G 2.3M 107G 0% /v0 
/dev/ad6s3e 116G 2.2M 107G 0% /v1 
/dev/ad8s3e 116G 2.1M 107G 0% /v2 
/dev/ad10s3e 116G 2.1M 107G 0% /v3 

I know what calls to make to get the "Size" and "Avail" columns. I'm not sure what df does to get the value for the "Used" column. Anyone know how df gets this value?