Your clients and handler hire you because they know they can trust you to be discreet. The best run is one that the target never knows happened. Drawing attention or getting caught will endanger your reputation, pay, and lives.
That is of course, easier said than done.
When things go wrong and draw attention, you’ll acquire Alarm and Heat. As these escalate, so too will consequences and challenges.
During a job, a significant part of your efforts will go to managing your alarm levels and working around collected heat.
Alarm
Alarm represents the active state of enemy awareness of and concern over player character actions during a job.
Alarm is added in 3 ways: pre-existing circumstances such as heat or scenario-driven alarm, alarm pool rolls, and some character actions may directly add to the alarm state:
Shooting a firearm within earshot of NPCs
If an unconscious or dead body is discovered
Failing a critical deception or ID check
Being spotted on a camera in a place you shouldn’t be
etc.
It is the DM’s decision if and when an action triggers direct alarm instead of raising the alarm pool. Generally the difference between the two is whether there is only a chance or certainty of the event being noticed.
During a job you are able to take actions to reduce alarm. As when reducing the alarm pool, players must state what action their character takes, and this action needs to be relevant to the current situation. (For example, spoofing camera footage to show an empty hallway will only reduce heat until the guards have spotted you in that hallway.) NPC’s may vary in their base suspicion, which may impact how easily they can be calmed down after an alarm.
Alarm tiers are locked. Once alarm has progressed to a tier, it cannot be lowered below that tier, though unique circumstances may limit the impact, or prevent an alarm from escalating to the next tier. (For example if only 1 guard is on site and that guard has been knocked unconscious, there is no one to actively search. Alarm may still progress toward the next tier, but depending on the ability of other forces to observe you, you may no longer be limited to 6 second initiative passes and it may not be possible to trigger escalation to outside forces.)
When possible, it’s generally a good idea to reduce alarm as much as possible before leaving a site.
Tier 1
Local Heat +1 after job
Tier 1
Active Alarm, enemies actively searching, initiative triggered
Tier 2
Local Heat +2 after job
Tier 2
Escalation, outside forces coming
Tier 3
Local Heat +3 after job
Tier 3
Dragon or nuke deployed
Some DMs may choose to keep the alarm level secret to better simulate the inability of players to see and know the things enemies know.
When playing with this rule, the players will still be able to see the size of the alarm pool, but the DM rolls and tracks alarm secretly like they do for Heat. The DM communicates changes as they are relevant to player actions, such as when initiative needs to be rolled. Players can attempt a perception check to observe the behavior of NPCs to find out information about the current alarm level.
When an individual scene ends, the alert state and unresolved circumstances roll over into Heat.

Heat

Heat represents the state of enemy paranoia and concern over security outside of a specific scene.
As heat accumulates, it will increase the base alarm, impact prep and downtime safety, and add additional challenges during jobs.
Heat will automatically reduce over time. If no additional heat is added for 4 weeks in game time, the heat will reduce by 1.
During prep and downtime you are able to take actions to reduce heat. As when reducing alarm, players must state what action their character takes, and this action needs to be relevant to the current situation. (For example framing a competitor power player for a successful job or returning stolen goods) NPC’s may vary in their base suspicion, which may impact how easily heat can be reduced.
Heat is tracked both locally and globally. As many factors affect Heat, players generally do not know Heat levels, though they may be informed of Heat-related consequences such as additional security presence or new security measures.

Local Heat

Each major power will have a local heat meter. This meter affects alarm and the amount of security on site at locations controlled by this power. As a power grows more concerned about attempts to interfere with their business, they will escalate their response.
Local heat is added in 3 ways: NPC or narrative events, Alarm rollover after jobs, and Character actions during jobs such as:
Harming or terrifying NPCs
Leaving a dead body behind
Damaging property
Generally any evidence of an unauthorized presence or crime will result in accumulated heat.
Local Heat Meter
Tier 1
Additional passive security = cameras, sensors, laser tripwires, etc.
Tier 2
More security personnel
Tier 3
Expert security personnel and mechanized response
Tier 4
Castle doctrine = Total location lockdown. Contract issued to neutralize player team.

Global Heat

Global heat reflects how much risk your characters have while moving around the city and your neighborhood.
The rich and powerful aren’t always eager to share the news that they’ve been had. This means that local heat does not always trigger global heat. Instead, global heat is triggered by:
Local heat with the power that controls your neighborhood (halved)
Any job alarm reaching the 3rd tier
Any local heat meter reaching the 3rd tier or higher
A violent crime reported to the local police or discovered by unaffiliated NPCs
A violent crime independently leaked to the media
Generally any public evidence of a crime will result in accumulated heat.
Global Heat Meter
Tier 1
Raised overall suspicion - all NPCs have +1 to innate suspicion
Tier 2
Difficulty getting restricted weapons, improving contact relationships has *2 cost
Tier 3
Regular citywide ID checks, public bounty issued
Tier 4
Faces and IDs burned, countdown to hideout raid

Identification

Every character has an Identity Number, or ID. This number is tied to a permanent record in at least one power player’s databanks that includes identifiable biometric data. This SIN is required to open a bank account, enter restricted spaces, get licenses for gear and actions, and more.
Luckily for runners, power players don’t like sharing information, which means that other power players generally have very little record for characters beyond basic biometric data. As you interact with these power players, they will add information to your record.
Runners are able to exploit this gap to build or borrow false identities. While your ID is always tied to your biometric data, the information attached to that identity is flexible and can be hacked and edited by identity dealers. When editing an ID, you are always only adjusting the data associated with a specific power.
Leaving behind biometric evidence at crime scenes will result in your actions being added to your ID. Initially this will only impact the information known by the power players that responded to the scene. If local heat spills over into global heat, evidence of the triggering crime will be shared with any allied powers.