Disk layout is slightly different on FreeBSDs i386/AMD64 then Windows or Linux.
One 'slice' called a primary partition in normal speak, is identified as driver disk# s slice# -- i.e. ad0s1 -> first pata disks first slice or 'primary partition'
Within a slice, FreeBSD can place a bsd 'partition' -- kind of like how dos extended partitions work but much better, a letter is suffixed onto the slice number, i.e. ad0s1a read right to left -> partition a, slice 1, drive 0, 'ad' device (basically ide/ata disks).
Default for PC-BSD is one slice containing two bsd partitions: a and b with a mounted as the root (/) file system and b attached as swap space.
The installer allows this to be customized (including a checkbox for encrypting swap :evil: ).
My (PC-BSD v1.5.1) laptop for example looks like this:
Terry@dixie$ df -lh 20:41
Filesystem Size Used Avail Capacity Mounted on
/dev/ad0s1a 9.7G 316M 8.6G 3% /
devfs 1.0K 1.0K 0B 100% /dev
/dev/ad0s1f 19G 5.4G 12G 30% /usr
/dev/ad0s1e 19G 239M 18G 1% /usr/home
/dev/ad0s1g 496M 133M 323M 29% /var
/dev/ad0s1h 22G 42K 21G 0% /tmp
linprocfs 4.0K 4.0K 0B 100% /compat/linux/proc
Because I created the 'f', 'e', 'g', and 'h' partitions in the FreeBSD slice during the installation.
What DragnLord means is, in such a layout it's possible to not allocate enough space, for example on my system /usr is over 5GB, it wouldn't be possible if ad0s1f only had 500MB of space allocated for itself.