There are 5 Difficulties to choose from in each Mission (Easiest to hardest): Rookie, Professional, Operative, Elite, and Legend. Missions completed at higher difficulties usually mean more income and EXP, but at the cost of being more difficult to complete. Entry Point has varying levels of difficulty to tailor game play to your skill level.
Rookie (Level 1+)
Rookie difficulty is indicated by a single white star in the lobby. Completing a mission on rookie or higher for the first time will award a crimson Badge for the respective mission.
On Rookie difficulty, there are few guards and cameras. Detection rate is very slow and NPC's will react sluggishly. You will not be charged for any equipment when deploying. Enemies spawn slowly, deal minimal damage and have very low health. Two squads deploy and special enemies are absent. The location of your teammates can be seen through walls, indicated by a white dot. Being close to teammates will change the dot to their username.
Professional (Level 2+)
Professional difficulty is indicated by two white stars in the lobby. It is very similar to rookie, most NPC's stats like detection or health are only increased by a small amount.
Professional difficulty also has a low amount of guards and cameras. Detection rate is slightly higher and NPC's will react faster than on rookie. You will be only charged half of your loadout cost when deploying.
During loud more enemies spawn quicker and are stronger. Special units like Shields and Aegis spawn when going loud. Enemies also throw flashbangs on professional+. Teammates can be seen through walls but only at close range.
Operative (Level 2+)
Operative difficulty is indicated by three white stars in the lobby. Completing a mission on Operative difficulty or higher for the first time on Stealth method will award a silver badge for the respective mission.
Operative difficulty has a modest detection rate with a balanced count of guards and cameras. Enemies have decent health and damage. Special enemies spawn more frequently and teammate locations cannot be seen through walls but the white balls will appear once seen.
Elite (Level 20+)
Elite difficulty is indicated by four white stars in the lobby. It is harder than Operative difficulty and offers quite a challenge for most players. It is generally considered the game's "Proper Experience" difficulty-wise.
On Elite difficulty there are more guards and cameras. Detection rate is fast and NPC's will react more quickly. You have to get closer to guards for intimidation and they will shoot you much faster upon detection. Enemies spawn frequently, have high health, deal high damage and strafe more often. Snipers shoot quickly dealing high damage, and there is no teammate indicator like on lower difficulties.
Friendly fire is enabled on Elite difficulty, but will not deal much damage to teammates.
Legend (Level 40+)
Legend difficulty is indicated by four red stars in the lobby. Completing a mission on legend without raising the alarm and killing anyone for the first time will award you a golden badge for the respective mission. It is the hardest difficulty and can offer a very strenuous challenge for most players. It is advised to reach the maximum level and familiarize yourself with the mission before attempting this difficulty.
Legend difficulty has the most guards and cameras, sometimes in special spots that can catch you off guard. You can only disable 2 cameras, unlike the usual 4. Detection is very fast. NPC's react about 4x faster than on Rookie and their range of detection is very far. The intimidation range is small and guards immediately shoot you if you're detected. Normal enemies and even Shields spawn very rapidly in large numbers, move around fast and strafe unpredictably, are very hard to kill and (Especially in groups) output massive damage. Even a single enemy can kill you if you aren't careful and/or lack the proper armor. Aegis Units and Shredders spawn in pairs. Snipers do not use lasers (Except in The Financier) and deal insane damage.
Friendly fire is also enabled on Legend difficulty, and unlike Elite difficulty, deals much more damage.