N/O (Normally Open) and N/C (Normally Closed) are terms defining the position of the reed switch (in door contacts and motion detectors etc). Whether the switch is ""open"" (no continuity) or ""closed"" (continuity present) depends on the magnet placement.
If there is no magnet next to a Normally Open contact, there will be continuity until the magnet is near.
If there is no magnet next to a Normally Closed contact, there is no continuity until the magnet is near.
Most hardwired devices used for Alarm Control panels use a N/C or Normally Closed switch.