A “diagram dungeon” is a 2d grid of square cells, with gaps between them. You can write and draw in them and between them, and you can also easily refer to them by their coordinates.
The rooms do not have to be to scale, and the corridors that connect them (that you can draw in between the squares) certainly don’t have to.
For example, you can indicate that there’s a 15 foot corridor between A1 and B1, and that B1 is a 50 long hallway, while you can indicate that A2 and B2 are connected by a 40 foot corridor and that B2 is a 25 foot room. (To the extent that you’re even listing sizes; you usually don’t need to.)
You can use separate sheets for separate levels, or also indicate vertical movement or vertical space.
Now, these aren’t polished enough for publishing or even for showing your players, but they are very clear and usable for you as a DM.
I’ve found that geomorphs can work well with diagram dungeons, especially if you have labeled geomorphs: you might have what’s primarily a diagram dungeon but you can also indicate that 2x2 set of cells is a specific geomorph, or even a single cell.
Of course, you can also have cells leading off into other types of maps, flowchart maps, detailed hand drawn maps, anything.
It’s definitively not necessary to combine with other types of maps; if all I had the rest of my DMing life were diagram dungeons, I’d be perfectly content! It’s just that the fact that they combine so readily with other types, should you wanna do that, is another advantage of diagram dungeons.
Red Tide from Sine Nomine was advertised as having something called “diagram dungeons”, among many other things, and it was the reason I got the book. I was disappointed at first, because I was very well familiar with using super abstracted flowcharts for dungeon maps, and I expected a huge step up from those, but this seemed to me to be a step down, to be the worst of both worlds. All the limitations of spatial layouts, very cramped space for drawing or writing, and the feeling of “this is nothing special”, “really, is this all?”, “it seems so obvious”. They weren’t described in the ad copy so I don’t know what I was expecting.
I think I even said as much in some comment on some other review or blog. Huge regret. Because as time has gone by, I’ve started really appreciating their speed, clarity, usability, and balance between consistency and flexibility. I’ve only used them a handful of times but those sessions turned out awesome. They are a step up from flow chart dungeons. It’s just that they’re so simple and clean that that’s not readily apparent.
The book overall is great; it has lots of other tools, site tags, it even has another quick dungeon generating method! (One that’s similar to Hero Quest or Cyberpunk V3.0, you cordon off rooms from a flexible base layout.)