Flanking is a game mechanic in XCOM 2.
Flanking occurs when a unit is targeted while being out of cover. A flanked opponent will have a yellow shield icon displayed next to their health and will have an enemy type icon colored yellow in the GUI when being targeted.
Flanking increases the attacker's critical chance, and thus potentially increases the damage output (depends on the weapon). XCOM units receive +40% critical chance bonus, while enemy units receive critical chance bonus depending on the game difficulty: +33% for Rookie and Veteran and +40% for Commander and Legend.
Note: certain special ability attacks set the critical chance to absolute zero (i.e. there is no critical bonus damage on such attacks, e.g. Reapers' Banish), and some special ability attacks do not have critical bonus damage at all (e.g. Skirmishers' Whiplash may show a critical damage notification but the damage cannot exceed the usual value of 6 for organic and 12 for mechanical units).
Some units cannot be flanked, e.g. Archons. [explanation needed]
Some units don't receive critical chance bonus when flanking, e.g. Psi Zombies. [explanation needed]
In some DLC-specific missions certain enemy units have different critical chance bonuses, e.g. Neonate Vipers receive +10% critical chance bonus on Rookie/Commander/Legend and +33% on Veteran difficulty settings. Also some units receive Aim bonus when flanking.
