Ah, the man is using map labels the way I do....
Supply is pretty simple once you get the hang of it, but definitely there is some level of art to it in understanding drain v supply. There are some automated features in cities and OF that keep most of the micromanaging at bay, and yes that means your used supply has gone towards staving off drain effects. When this starts to happen, its a sign that production drain penalties are getting closer too being enforced. When using supply, the Drain value itself becomes fuzzy a little in that it does not account for how much supply could be applied next turn.
I highly recommend reading the section on production and supply in the manual, as a lot of details are in there about how it is working.
This post makes me think I need to add a message in the production report that tells how much supply was used to maintain drain that turn. Currently you do not get that message.