Explained- Microsoft Windows OEM vs Microsoft Windows Retail -
Microsoft Windows OEM version (Original Equipment Manufactuer) is a specific type of product user licence which is limited by the amount of technical support you get which in case of Windows OEM is always provided directly by the system builder, i.e. by the one who bought and installed the 30% cheaper OEM version of MS Windows.
On the other hand, the more expensive Microsoft Windows Retail packaging includes dedicated Microsoft Windows Technical Support. Don't get tricked by allowing some on-line PC shop to sell you a not pre-configured Windows PC and make sure what type of legal licence comes with your new PC!
It is not illegal for an end-user to build his own PC with Microsoft Windows OEM operating system, however he then becomes the system builder, and the technical support then lies with him. At the era of online security, please bear in mind that one may find himself running not quite secure and hence easily exploitable Microsoft Windows operating system version.
Scenario 1: a seller of PCs sells a new Window 10 PC, where the seller bought and pre-installed the cheaper OEM Windows for it, hence becoming the system builder. Technical support lies with the seller of that PC.
Scenario 2: An end user bought the more expensive retail packaging for Microsoft Windows (or even decided to purchase the retail version with his new PC). The technical support lies directly with Microsoft, any technical support by the actual seller of that PC will be very likely limited to hardware-related issues only.
And yes, you can install the retail version on all different PCs, as long as you always delete the previous installation from the previous system