You're seeing just the macro assert_pipeline_notified, go back to Membrane.Testing.Assertions module for more information.
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assert_pipeline_notified(pipeline, element_name, notification_pattern, timeout \\ 2000)

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Asserts that pipeline received or will receive a notification from the element with name element_name within the timeout period specified in milliseconds.

The notification_pattern must be a match pattern.

assert_pipeline_notified(pipeline, :element_name, :hi)