When a unit is described as 'on scene,' which code is used?

When a unit is described as 'on scene,' the appropriate code used is 'OS.' This designation indicates that the responding unit has arrived at the location where they are required to respond. 'On scene' refers to the moment when emergency personnel or units reach the specific site where an incident or event is taking place, and they are ready to engage in whatever actions or assessments are necessary.

The other codes listed serve different purposes. The 'IA' code often refers to 'In Area,' meaning that the unit is in the general vicinity but not necessarily at the specific location of the incident. Similarly, 'NO' represents 'No operator' indicating that there isn't a clear communication line established, and 'PS' typically means 'Personal Service' which relates to a different context of dispatching. Therefore, 'OS' specifically aligns with indicating presence at the incident site, which is why it is the correct choice in this scenario.