The spec seems to use a faint middle dot character to represent a blank space, but this isn’t explained anywhere.
There is a sentence about the use of right arrows to mark tabs, in the About this document section.
I would expose a separate section for a legend, if you will, that makes the symbol use more salient (the sentence about Tabs is sort of buried at the end of that section).
Re: tabs, I had no idea that those arrows represented tabs at first. I don’t know what’s most common, but the right arrow is not listed on the Wikipedia treatment of visual representation of the tab character.