Compatible computers, or rebranded
In this page I gather documents on compatible computers.
The "compatible computers" in the 1990s were computers not branded Olivetti or IBM or other big manufacturer.
In 1990s it was easy to buy some computers in Japan or something, and rebrand it with your logo. Burocracy was very soft, certifications almost non-existent.
The customs procedures were longer, but that was all.
It was Windows that permitted this. Before this, IBM, Olivetti and other tried to impose their operating systems, that was sold only installed on their machines. Doing so, you could only buy a PC from them, because you cannot create your operating system.
With Windows you could buy only the operating system and use it on whatever machine you wanted, breaking the monopoly of the big players of the market.
Here there are some documents about that first era of compatible computers.