Why does bsdlabel autogenerate the 'a' partition?

Hi

Why does the bsdlabel program autogenerate the 'a' partition?  If
I edit the disk label removing all partitions except for 'c' and
then save it, a subsequent read of the disk label shows an 'a'
partition has been made covering the whole disk.  Even if I make
another partition using the whole disk, it still makes 'a', which
has to be deleted to create other partitions because of the resulting
overlap.

Ian