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

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

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

{:ok, pid} = Membrane.Testing.Pipeline.start_link(%Membrane.Testing.Pipeline.Options{
  elements: [
    the_sink: %Membrane.Testing.Sink{}

You can match for exact value:

assert_sink_caps(pid, :the_sink , %Caps{prop: ^value})

You can also use pattern to extract data from the caps:

assert_sink_caps(pid, :the_sink , %Caps{prop: value})