Create a new full backup after ... parameter not available

Hello,

I am using ATIH 10.0 build 4942 on XPSP3.

The help file for this software says: "Create differential backups - having chosen this parameter, check the Create a new full backup after ... parameter and specify number of differential backups, after which a new full backup will be created. The first time a full backup will be created. Next backups will be differential until the specified number of differential backups is reached. Then, again a full backup and a set of subsequent differential backups is created, then again a full backup and so on."

However, when I select the differential backup option, there is no check box for "Create a new full backup after ... parameter". This is an option I would like to use so why is it not there when help says it is?

Thanks,
Bob