The Metro Police Force consists of (on most servers) groups called divisions. They each have a separate role in running the city. Divisions are typically quite different between servers since there are no "official" divisions. In fact, Divisions is mostly a made up thing, considering they do barely anything of value and are usually there to make said player feel special and/or more cool. Divisions militarize the cops. They should not be militarized, they should feel like paramilitary thugs. Remember that they do random house raids and fire their MP7s from the hip, remember that they beat citizens for no reason. They are not soldiers and should never be thought of as soldiers. At best they should storm a room and beat the shit out of defenseless people, because they are thugs to scare the populace. The most they should ever do is guard a Canal room (aka super minor Combine outpost) or shoot three unarmed Rebels trying to build a pipe bomb in their room. Actual assaults and military work is exclusively the job of OTA.
Authenticity makes it feel more like HL2, i.e. what players want to roleplay.
Razor is a defense/offense division for anything regarding Rebels. They receive an MP7 and are handpicked by Division Leaders.
Union units patrol and maintain civil order. If any Citizens break any regulations, they will mainly be handled or taken care of by Union units. Union Division Leaders can lead lower ranking units to non-patrol areas (e.g. slums, sewers).
Helix is a Medical Division, responsible for giving medical assistance to other units, No matter if it's OTA or CP/MetroCop. In order to be in the Helix Division, a unit must have extensive and medical skill and training. Most Helix units remain inside the Nexus, leaving only if you are being a Shield to them, and only if you assistance in Sweeps and Patrols.
Grid is a mechanical division. They make Combine Cameras and City Scanners. They will sometimes patrol and will be called in for backup if needed.
Jury is used for interrogating Anti-Citizens. They will be sometimes sent outside for patrol or guarding Hardpoints, but this is not often. Interrogation will be used if citizens are: Anti-Citizens, Level 5 Contraband, Level 6 Contraband, Level 7 Contraband, and if they have below/higher of 9mm ammo.
Nova are MPF Unit guards that Patrol the Nova Prospekt side of the Nexus. They try to keep all/any prisoners inside of their cells, and not let any of them escape. They aren't really sent outside of the Nexus, only if a Judgement Wavier is in effect.
Zone is a guarding division. They specialize in guarding Hardpoints, checkpoints, and restricted areas. They are to protect these areas so no Citizens can pass.
Mace is a special division. These units specialize in Special Operations, and breaching tactics. These units aren't really sent outside to patrol or to guard Hardpoints. They are only sent outside of the Nexus, but are sent outside only when a Judgement Wavier/Autonomous Judgement is in effect, and if they are needed in sweeps or Rebel Base raids.
Spear is a division that specializes in torture and torture tactics. They may torture: Anti-Citizens, confirmed Resistance members, prisoners, and Citizens with a large amount of Contraband. They are mainly used for Amputations.
Judge is a higher ranking Jury division. They have more questioning tactics, can torture sometimes (but not that much), and they mainly remain inside of the Nexus. They can also be used for Amputations. You can only join this division if a higher ranking Judge unit requests that you join.
Shield is a high, advanced division. They are trained with the ways of defense and protection. These units are appointed with defending City Officials and other Units, such as the CWU Manager, the Minister of Health and the Helix Unit when available.