(Updated in 2024)
From inside out: pcb-rnd is a layout editor software. It is part of the Ringdove suite, among with sch-rnd, edakrill, etc. The suite is part of CoralEDA, which is an EDA ecosystem effort, which in turn has two rings.
It's probably easier to understand it looking at the following diagram:
The rings are, from outside to in:
The main difficult part is to see the difference between the Suite and the ecosystem. The key is realizing how tightly coupled the goals are:
On the other hand, the ecosystem contains tools from various sources, developed by different people (with small or no overlap with our developers). Development goals and styles are rarely the same. But at the end, tools within the same ecosystem have some practical connection for the end user.
The following drawing shows the relations between pcb-rnd, the suite and the rest of the ecosystem (without dividing the ecosystem into two rings this time):
Thus the connection between ecosystem tools are much weaker then the connection between the tools within the same suite. The ecosystem model is further divided into two rings, depending on whether the participant is willing to join the ecosystem (tight ring) or just lured in (loose ring). Paths to and from loose ring tools are more likely to break as developers of those tools won't consider ecosystem aspects.
Please note: being free software is orthogonal to this splitup. For example gEDA/pcb or lepton EDA are both free software, but they are in the loose ring becuse they don't do anything for interoperability within this ecosystem. On the other hand, there's no actual example yet, but there might be a proprietary software or service that decides to use the tEDAx file format, which effectively makes it part of the tight ecosystem.
|printed circuit board editing
|"cloud" - web based user data lib (footprints, symbols, etc.)
|Trivial EDA eXchange - cheap-to-implement file formats for interoperability
|generic project manager
|PCB CAM file viewer
(gerber, excellon, etc.)