Hi,
They aren't talking about distinct units as such, although the wording is... odd. Nice try though.
The initial amount and each 5 GB after is a threshold, so once you pass that threshold you get charged. In the end it's up to you whether you decide to pass the next one.
And it'll be in your T&Cs.
Get Networx if you want to keep some form of track, although note that it won't count anything like mobiles, iPads, consoles, etc.
Hope that helps.
Cheers.
Dave A
