I didn't imply an ASCII mockup by my example. That grid is a way to declare, in form of self-depicting text, where the major components of the diagram go relative to each other. Imagine that every one of the components is a group, and each group contains half a dozen nodes, just as many arrows, and some may even have their own hierarchy.
The point is to pin major high-level parts of a complex diagram, so that as you continue to add, remove or modify the details, the rendered diagram doesn't look completely different every time you change something. This addresses my major issue with PlantUML, in that with any nontrivial diagram, even the tiniest change - such as adding or removing a link, or reordering neighboring nodes, can cause the diagram itself to completely flip, or spaghettify itself.
The point is to pin major high-level parts of a complex diagram, so that as you continue to add, remove or modify the details, the rendered diagram doesn't look completely different every time you change something. This addresses my major issue with PlantUML, in that with any nontrivial diagram, even the tiniest change - such as adding or removing a link, or reordering neighboring nodes, can cause the diagram itself to completely flip, or spaghettify itself.