How does Graphic Banks work?

I'm a bit confused by this. There are three graphic banks you can use in the left menu in NESmaker, and there is a drop down menu at the top of the map screen. Still I can't make sense of this. How do I change graphic banks if I for example want to place assets from graphic bank 2 in a screen?

The Bank drop down menu doesn't seem to do anything...

you have to select the bank before you make a screen.
when you make a screen, the selected bank is then used for that screen and cannot be changed.

the banks work by using 6 BCKCHR files per bank. you have 18 of them in your graphicsassets folder, the first 6 are bank 1, the next 6 are bank2 and the last 6 are bank3.
similarly, 10 BCKSSCHR files (screentiles) exist in each bank.

Thanks Mugi. Great explanation as always!

So I can use assests from multiple tilesets on each screen like this?!?

Make sure you try to plan your tile sets. Like Mugi said, the screen bank cannot be changed. You'll find yourself in my position, wanting to restructure your tiles so that more similar assets can be used together, with 30+ screens that need to be recreated all over again.