When you BYPASS a zone in the security system, that zone is disabled, ie will not cause alarms, until the system is next disarmed. The reason could be a faulty zone or a wndow that you want to open and close when the system is armed. Remember that BYPASS is temporary until the system is disarmed. A 24 hour zone cannot be bypassed.
When you arm a system and there are zones which are open, you can press # to Force arm the system. This means that ALL open zones which have a security function are automatically disabled, allowing you to arm, withut ahving to bypass each zone individually. Again it could be due to a faulty zone or sensor. Perhaps a window elsewhere in the house is open, so you can force arm and then close the window later.
An important difference is that a force-armed zone is ignored or disabled until it is closed again after which the zone is enabled ie violation WILL cause an alarm
Bypassed zones can be open and closed without causing an alarm, until the system is disarmed.
You can force arm a system when doors or windows are open, but you should NEVER forrce arm a system when motion detectors are repeatedly activating because of movement in the premises. When the motion detector restores and reactivates it is force armed again. This will prevent the system from arming if a zone is constantly closing and opening after you do the force arm. Even if the system successfully arms, the motion will cause an alarm. This situation may sound unrealistic but it may occur during testing and commissioning a system where there are other contractors in the premises, and someone is trying to arm the system to test it. The zone must be BYPAASED not force armed in this situation