Anyone knows wich computer ID MT4 uses for activations of market products?

Where can I see my purchases? 
https://www.mql5.com/en/forum/9950/page70#comment_2564931  

How to Install an Earlier Purchased Application
https://www.metatrader5.com/en/terminal/help/market/market_buy

Thank you very much for your response. However, some beginner tutorials do not answer the questions. 

I am asking which machine ID is used by MQL to limit the access to a given computer. Is it the hard disk serial number? Or some other machine ID I don't know of yet.

it is per hardware and per OS (I am sorry if I understood it wrong).

No problem, and thanks for the additional answer. :)

I assumed it was hardware, but not sure if it is the hard disk serial number or some other stuff I never heard of. 

This OS relationship will bring a lot of troubles when traders will have to move to Server 2016/2019 when 2012 R2 is out of support. From that point on each major/minor Windows update (twice a year?) requires an additional activation (as it is required with Windows 10 too). Hopefully Metaquotes will provides a solution therefore.