What does a black triangle mean?

[Lollipop's Mom]

On the deck plans, some rooms have little black triangles, but I can't find anything that lists what this means!

 

[mmouse37]

On the deck plans, some rooms have little black triangles, but I can't find anything that lists what this means!

Those are rooms that CONNECT to the one next to it (have an interior door so you can pass from one to the other without going in the hallway). The points of the triangle show the connecting rooms.