Here’s the feature announcement.
Often when you use a datasource you use a message as well. For instance, if you specify, “Current user’s email”, you are using the “Current user” datasource and the “'s email” message. Similarly, you might use a source of text as a datasource with a message like “contains”. This feature allows you to search the app for places you’ve used a specific message.
Since messages are specific to a certain data type, when selecting a message to search by you first choose the relevant data type your message operates on. For instance, if you’re looking for an instance where you use the greater than operator, you’d select number as the type before selecting the “>” message.