An external siren that can be heard when people are moving around outdoors, telling them to shelter. Manned buildings with building controllers to collate information to a central point, depends how big your workplace is. It can be quite straight forward really. I am hoping by the word invacuation you do mean shelter? as that word is not in the English language!

These are quite often used in hospitals or residential accomadation, voice alerts telling people to prepare for evacuation or different tones to alert to a possible vacuation being required. It gives staff and service users the opportunity to prepare for full evacuation. As ;ong as everyone understands thier role in the procedure they often work well, the procedure needs to be tested at least annually, but some places are required 2-4 times annually.

Talked about holding until they needed to be evacuated. But point taken, not sure about alert tones to tell you to stay in unless chemical, oil refinery etc. other than that the situations for most of us would be adhoc.

For chemical plants, refineries, etc. an outdoor 'toxic alarm' siren is not uncommon, obviously it's a different sound from other emergency alarms, and all site personnel + visitors need to understand the correct response. In addition to moving quickly into a building, the alarm response within the buildings has to be effective, e.g. close windows (if they can be opened) and shutdown ventilation fans - normally that would be an automatic link when the alarm is sounded. If there are residential areas nearby that could be affected, sometimes the procedure is extended to these by informing residents what to do. But that should only be on older sites, as local planning controls should prevent occupied buildings within the area that could be affected by a major on-site incident.