Hi,
JabRef is a modularized app that uses jlink to create a custom runtime:
jlink is a tool that generates a custom Java runtime image that contains only the platform modules that are required for a given application.
Such a runtime image acts exactly like the JRE but contains only the modules we picked and the dependencies they need to function. The concept of modular runtime images was introduced in JEP 220.
The idea is that you no longer need to install any JDK/JRE on your system and thus avoid having troubles with programs using different java versions etc.
If you really want to package it in another way, you can maybe take a look at the arch linux packaging
https://aur.archlinux.org/packages/jabref
build/image contains all the necessary stuff to run JabRef:
build/distribution is basically packaging the image for distribution. e.g. as deb or app file (I am using mac)
The one build/distributions (the one with s) is only some intermediate folder.
PS: Do you have a link to the solus repo stuff?