CONTROLFILE FORMAT '%F'

I hope that someone can explain to me the below scenario.

Thanks

In RMAN when I type:

show all;

I get

CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO '%F'; # default

When I issue :

backup current controlfile format 'ctrl%F.ctrl';

I get error

ORA-19715: invalid format F for generated name

My question is why I cannot use the %F variable with the format clause in the backup command or Iam I doing something else wrong?