Sector 16 is the last sector in the game, it is rainbow. Its difficulty is the same as Sector 10th, so there is nothing special. Getting a Vehicle is recommended. Alternatively, using S.A.M. significantly increases the likeliness of seeing this sector (however, this is skill-based). If another sector is reached, that (and all the additional sectors) will all be labeled Sector 16 (probably because the sector number values are 4-bit).