Must root be on slice 'a'?

[Leif Neland]

http://geodsoft.com/howto/dualboot/freebsd.htm
writes this:To install a new FreeBSD system, you must create a new '/' file system at a minimum. If you do this in a slice with an existing '/' partition the previous system will become inaccessible. You can normally tell that there is a previous '/' mounted in the current slice, if any existing partition name ends in an 'a'. 

Does this imply that I must rename my slices, that I can't boot from /dev/ad1s3e ?

Leif