From what I read they are power and ground (return power). Some grounds are also for sensoring.
How the 6-8 is cabled I do not know. But some connector cable are BOTH 6 and 8 where the two extra are detachable (on a separate plastic thing that snaps onto the 6 part) and becoms an 8 IF used, or a 6 if not used… So I presume you can alter your cable… but I DO NOT KNOW!
The short cables are converters. Many PSU’s have undetachable cables and 8 pin is newer than 6 pin, so those need to be split.
Apparently the reason for 8 instead of 6 is to allow more power through thin cables, thus more thin cables, so they don’t run hot. (How Apple let you charge a laptop through a usbc is beyond me as those are thin…)
Edit: by looking at that picture it appears that the two extra are ̶j̶u̶s̶t̶ ̶p̶o̶w̶e̶r̶,̶ ̶n̶o̶ ̶g̶r̶o̶u̶n̶d̶. So read up 
EDIT 2: It’s the other way around, those to apparently are ground, not power. Colored ones are the 12V ones.