number system

NOUN
13 LETTERS · STARTS WITH N

number system is a valid 13-letter word worth 21 points in Scrabble and 25 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
N1U1M3B3E1R1S1Y4S1T1E1M3
1 + 1 + 3 + 3 + 1 + 1 + 0 + 1 + 4 + 1 + 1 + 1 + 3 =21 PTS
DOUBLE WORD → 42TRIPLE WORD → 63
MEANING

What does number system mean?