When you only have one device inside your backup storage group and one device inside your mirror group, it does not matter which mirror mode you select.
But, when you have a mirror group that consists of more than one device/location, you can choose to correlate the mirror storage devices to a corresponding backup storage device/location; which is referred to as an Interlocked Mirror. Or, you can choose to use the mirror in an Ad-hoc mode, in which the mirroring process will replicate the files form any of the backup storage drives to any of the mirror drives based on available free space.
The goal of using the throttle option is to give you control over the speed at which the mirroring process occurs. By default, the mirroring process will consume as much bandwidth as available between the Backup Server and the mirror device. If you want to prevent the mirroring process from consuming all the available bandwidth, use the "Limit the mirror update rate" option and specify a rate appropriate for your connection. For example, if your mirror device is off-site, you may want a throttle speed that is half of your Internet link speed, to prevent the mirror process from consuming your Internet link.
Use the blackout feature to prevent the mirror from consuming excess resources during business hours. For example, if the mirror is located off-site, and you want to prevent the mirroring process from consuming your Internet link during business hours, you can set a blackout period to keep the mirroring process quiet.