A142: If you are the defender:
a) Classes that grant Uncanny Dodge stack to determine the minimum Rogue level needed to flank you. If the attacker has four more Rogue levels than the sum of your classes that grant Uncanny Dodge, the attacker flanks you and can make a sneak attack.
b) Non-Rogues can't flank you.
c) Same as a), since Assassin grants Uncanny Dodge they stack to determine the minimum Rogue level required to flank you.
A142: If you are the attacker:
a) Your Rogue levels must be four higher than the sum of the target's classes that grant Uncanny Dodge if you want to flank the target and sneak attack.
b) You cannot flank a target with Improved Uncanny Dodge if you have no Rogue levels.
c) Same as a)