The ECC Queue is the normal connection point between an instance and other systems that integrate with ServiceNow, such as MID Servers1. The ECC Queue displays input and output messages from and to MID Servers2. The actual XML payload that is sent to or from an instance is stored in the ECC Queue3. The ECC Queue also provides a connected flow of probe and sensor activity, as probes are sent from the instance to the MID Server, and sensors are sent from the MID Server to the instance2.

References:

1: ServiceNow Discovery Overview

2: ECC Queue Processing and Debugging, with “Discovery - Sensors” used as an example

3: Manage ECC Queue content for a MID Server