in short, that the more is known about you, the more you are controllable. this theory, in its own right, is not that extreme but it depends how stringently you apply it to our society.
the idea is that with more and more movements to create an insurance health culture, and a completely individualistic society, you are completely to blame for anything that goes wrong with you, and to blame for any ills you cause to others. Therefore if you can track all spending, you can say "you drink too much and eat shit food, therefore you're insurance is massive and you're not entitled to access certain societal privileges". And then you can start banning things and people simply cant buy them.
You could argue China have been doing this for years with 'society points' or whatever they call them, ranking citizens and so forth, and it depends how unbelievable you think that might be in our european societies. Some folks think it's very likely coming, some don't.
30 years ago this was dystopian storyland, it'll never happen, fairly crackpot stuff. Nowadays I would argue it's far less crackpot, but controlling transactions, denying you access to services and goods due to potential negative impacts on 'society' and services, seems a tad far reaching. i believe informal transactions would simply step in. But all the same, it's all becoming a bit more possible...