ServiceNow Flow waiting for another Flow

use case:

The flow/subflow can be configured to wait for a specific message from any source. It doesn't have to be limited to a REST API inbound payload or an inbound email. As long as there is even a notification/event that contains the payload we're expecting and the flow is configured to listen for it, it can resume execution when that message is received.

In our case, it simply having separate flows that don't have access to each other's execution context but still needs to communicate. One flow pauses and waits for a specific event, while the other flow (or another process) sends the expected payload once it completes its work. When the waiting flow receives the expected message, it resumes execution.

reference:

https://www.servicenow.com/community/developer-blog/sending-message-to-an-active-flow-from-outside-of-context/ba-p/3194739