The rewrite of the RRs (resource registries) started in 2013 or 2014 and landed in Plone 5.0. I would recommend using Plone 5.1, as various things have been improved there.

With the RR you register and deploy JS and CSS. You can organise dependencies, and optimise resources and number of requests. The resources are grouped into bundles, they are concatenated and minified. Add-ons can easily register their resources. Cache headers are set automatically.

In Plone 4 there were no formally defined dependencies, so that made it hard to manage. You just had a list, and that order was used.

In Plone 5, the RRs are based on plone.registry, RequireJS, LESS and the command line interface of gulp. Instead of LESS, a lot of projects have switched to using SASS now. RequireJS is also less popular. So there is still room for advancement.

The Plone 5 way solves dependency management, but it is complexer and harder to debug. I have had my problems with it, but usually it works quite well, and is a huge improvement over Plone 4.

The js/config.js file in mockup contains configuration on how the javascript in Plone should be built. In Products/CMFPlone/static/plone.js you can see how the plone bundle is defined, and what it requires.

You can still define legacy resources, which work like they did in Plone 4. They are wrapped by some code that temporarily undefines the define and require definitions, otherwise you get errors.

In Products/CMFPlone/static/plone.less all the needed LESS definitions are imported or defined, used for creating our CSS. LESS is very handy for defining for example a text color once and use it in lots of places. [In Plone 4 we did this by using DTML files.]

You can customise and override the plone and plone-loggedin bundles in your code, if you maybe do not need everything that Plone offers by default.

collective.lazysizes has a good example of defining resources and bundles. In its registry.xml it uses condition="have plone-5" to only apply this part on install when the site is Plone 5.

With ./bin/plone-compile-resources -b plone you compile the plone bundle resources. This is also possible TTW, but I recommend the command line tool.