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

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Asserts that Membrane.Testing.Sink with name sink_name received or will receive an event within the timeout period specified in milliseconds.

When a Membrane.Testing.Sink is part of Membrane.Testing.Pipeline you can assert whether it received an event matching a provided pattern.

assert_sink_event(pid, :the_sink, %Discontinuity{})