In Word, what does "single" line spacing actually mean?

If you look at the paragraph settings in Microsoft Word, there's an option for "Line spacing". This actually seems like a misnamed field, because it seems to control leading.

The default is single, but there are options for Multiple and Exactly. If you use exactly, you can approximate setting the leading value for lines (such as using exactly 15pt to get 12/15pt Arial).

What does the default value of "single" actually mean? Is it scaling somehow to the font? Using some value built into the font?