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Thanks. I was about to say... 'ahhh, now I understand'... but in truth I don't.

It's simple, really. The red dots indicate the frequency and spacing of samples, as has been said. More samples, more accuracy with bounced light, but longer render times.
Have a look at this link:
www.except.nl/lightwave/RadiosityGuide96/

It's for LightWave, but the idea is the same, and towards the bottom shows how the GI dots and guides help with determining accuracy vs. render time.