According to the description of the " Minimum PLC scan time " in unity pro : this number is just the recommended minimum PLC scan time necessary to support the current configuration. So that is the minimum PLC scan time for the NOC and the devices connected to it.
So for example if you increase the number of IOs/devices connected to your NOC ( in the DTM browser) you should notice that this number "Minimum PLC scan time" will also change.
So for example if you increase the number of IOs/devices connected to your NOC ( in the DTM browser) you should notice that this number "Minimum PLC scan time" will also change.