Flanking

Flanking grants +2 to melee attack rolls. This requires that both attackers be actively and dangerously harrassing the flanked opponent. Common sense applies. Your cat familiar cannot actively flank a soldier, it is not enough of a threat to the soldier.
This +2 is not cumulative with multiple flankings. If you are completely surrounded, you are only granting +2 flanking attacks. This +2 can be combined with advantage or disadvantage that has been granted for some other reason.