Rank Restrictions help players in undersized parties find similarly Ranked players to queue with. Undersized parties are parties of two players in a 3v3 Standard Match or 3v3 Standard Tournament. This feature does not affect parties of two attempting to queue in 2v2 or Extra Mode Playlists.
Players need to be within 3 Ranks of each other. This is so a 3rd teammate will be able to join a similarly Ranked team.
Example: If two party members are Platinum 2 and Diamond 3 (a gap of 4 Ranks), they will not be able to queue in 3v3 Standard. Alternatively, if they are Platinum 3 and Diamond 3 (a gap of 3 Ranks), they can queue.
Brand new players can queue with players who are ranked Gold 3 or below. For more info, check out: Rocket League Party Skill and Matchmaking.
This feature does not take Rank Divisions into account. For more info, check out: What Are Rocket League Competitive Ranks?