The Free Software Foundation (FSF) describes this copyleft aspect of the GPL in terms of "derivative works" associated with GPL-licensed software. When two components are related to each other in a derivative way, then the GPL says that the derivative must likewise be licensed accordingly.

So in this example, does win.bat simply execute commands to get win.com started? Is win.bat a glorified shell script wrapper? If so, then win.com would NOT be derived from win.bat. The cart follows the horse. But instead, if win.bat exposed some symbols or other binary API features that win.com was coupled to and depended on, then you could rightly argue that the win.com would be a derivative of win.bat.

More practical of an example, if a database is licensed under the GPL, clients that connect to the database using the socket interface do not constitute a derived work. Or components in a micro-service architecture do not necessarily need be licensed all under the GPL when a single component is.

Pluggable architectures are possible with the GPL. And of course, your interpretation of what exactly that means is subjective and requires case law to help understand.

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And to reinforce what the parent of yours is saying, the author in the original example can do whatever they want with the software, since they own the copyright for both the GPL and proprietary components.

The GPL is simply a license for non-copyright holders. It allows others to be able to use a piece of proprietary software without having to establish any additional authority with the owner. e.g. it's the means to convey how others can use the software and does not constrain the owners/authors of the software. Other licensing options may be available, if the copyright holder allows.