Okay, basically those little patches appear as spot lights if you shine a infrared light on them--even from really high up. So their basic purpose is two fold--prevent friendly air craft from dropping bombs on you and let friednly aircraft find you on an E&E scenario. Those of you who have seen this photo 
might recall that the most chilling words I've heard were the co-pilot of the AC 130 saying "We have troops in the open" me knowing that those troops we us and with images of friendly fire dancing in my head I was grabbing my mike button when the weapons officers comes up and say "They're ours" feeeyooo
anyway-- basically they are a code--that day we were wearing two on our left shoulders and one on our helmets--the number doesn't matter
cause the can fall off. If an enemy gets one likely they will not have a bomb dropped on them--basically, the purpose is to prevent friednly fire, so, someone could use them to escape but not likely
If there is cloud cover are they still visible? If the IR light can burn through the clouds the reflector can be seen--but if not no
Are they on various parts of your uniform in case you are laying on that arm or that shoulder is hidden? No. but if you heard a gun run coming in and thought it might be for you, you would be trying to change the pilots mind, to include making sure your patch was visible
Do most countries have this? No--most counrties don't fight at night or have the IR capability we have.
Can they tell different uniforms from aircraft by the infrared reflectors? No.