what is the point system about? the rarity?
Condition.
Generally, it's a 70-point scale, derived from the one used for coins.
70 is theoretical perfect and ideal; some people claim a 70 cannot exist, but some grading services throw it out.
60 and above is considered uncirculated-- lower numbers perhaps mean worse overall quality (likely poor cutting/centering/stock)
Below 60, you tend to see steps. 58, 55, 50, 45, 40, 35, 30, 25, 20, 12, 8, 4 are probably the most common.
I've seen even a note torn in two and taped back together command 8 points, though. I suspect in that case, they say 'well, it was a 45 if it was intact, so deduct something for the damage'
The prices I quoted are actual numbers based on one specific type (South Korea, 5000 won type of 1972) I've been looking at.