Often when we hear and read about OEM license we are actually talking about an OEI (Delivery Service Partners Original Equipment Installation).
A license that can be purchased individually to combine it with a PC. The license is linked to the machine and once activated it is possible to reactivate it only on the same PC. The OEI license is a license that must be purchased by the assemblers who will include it with the PC.
The OEI licenses are sold in the DVD package + label with the product key code (commonly called COA).
The support is operated by the PC manufacturer, not Microsoft.
The Windows 10 Pro OEI license allows for a legal downgrade to an older Pro version of Windows (Windows 7 Pro, Windows 8 Pro, Windows 8.1 Pro). For instructions on how to properly downgrade, just contact the PC manufacturer.
ATTENTION to those who only sell you the COA label, these only label licenses are not original. DSP OEI licenses are sold paired with DVD + sticker, while COA labels alone are not legitimate licenses and as such cannot be resold as legitimate Microsoft software.