If I understand what it's saying, it says that the "M decoder" is a 1 bit in the least significant position of the 3rd nybble. So if the 3rd nybble is odd, M decoder is on. The 3rd nybble being 7 as in the example means all three decoder bits are on. (as 4 + 2 + 1 = 7). That's assuming that ON = 1, OFF = 0, which is not explicitly stated but would be typical. It's possible that this logic is reversed (ON = 0, OFF = 1), as the description of Gamma Correction seems to imply: "Gamma Correction ON/OFF (0/1) (For NN5198)". Overall I would not call this the best written documentation ever, but that's typical of many Japanese products. With more experience you can sometimes guess what they're trying to say.
Many of the bits are described as "Reserved for internal using, must be zero", but notice how they're all the most-significant bit of their respective nybble. This means that the hexadecimal digits for each position do not take on the full range of 0-9, A-F, but are limited to 0-7. The only exception is "NTSC QSS" which when turned on would make the first nybble equal to 9 (8 for NTSC QSS + 1 for NTSC Decoder On = 9).