It only shows things you can walk on for the most part. The vast majority is there for tunnels and original and current intended purpose is for finding and getting to Portals. There are a few flat landmarks such as Nether Wart farms and the occasional chest/workbench to help with orientation. Portals are labeled with the same words used on attached signs, except a few mystery portals I hope to get some community participation to identify.
I've come up with a way to get text into it now, and spent most of last night and today getting the portal labels in. Hopefully won't be too long before I publish a first draft, and certainly look forward to suggestions and such.