[UK-CONTEST] VHF - Why no single operator 6 hour section?

[Edward]

Hello,
Having just done a session in the May 144 contest it appears there is a
section "missing".

If a group do the full contest they are in "O", if they do 6 hours only they
are in "6O". Sounds reasonable.

If a fixed single op does full contest he/she is in "SF", if only 6 hours
it's section "6S". Again quite OK.

If a single op goes out /P and does the full contest it's "SO", but there is
no specific section if he does only 6 hours. 
Does he have the choice of "O" or "6O"?
Or should there be a "6SO" section?

I'm a little confused!

Regards,
Eddie
G0EHV