To best answer this question we can look at two things: UNIX commands and what is in a .plist.
Starting out with a .plist, the following code is usually there
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
The CFBundleExecutable identifies the name of the bundle’s main executable file. For an app, this is the app executable. For a loadable bundle, it is the binary that will be loaded dynamically by the bundle.
launchctl unload some.plist will tell MacOS the key to locate the bundle’s executable file and 'unload' it or essentially kill it from the system.
This is telling it at an application level.
When using a UNIX command such as
kill <PID> it is in reference to a specific process.
Looking at what is a Process vs Application here is good reading
You can read further into what is in .plists and how they work over at Apple