A Link Rating (Link Number in the OCG) is a property exclusive to Link Monsters, in place of a Level. A Link Monster's Link Rating is equal to the number of Link Arrows it has AND Link Materials it requires.
As Link Monsters do not have Levels or Ranks, they cannot be used for Ritual, Synchro, or Xyz Summons; and any effects that would refer to the Level or Rank of a monster, such as those of " ", " " and " ", cannot be applied to a Link Monster. Since Link Monsters do not have Levels, cards such as " ", " " and " " cannot be used on them at all. Cards that modify Levels or Ranks such as " ", " ", and " " also cannot be used on Link Monsters.