You're seeing just the callback handle_process_list, go back to Membrane.Filter module for more information.
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handle_process_list(pad, buffers, context, state)

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Callback that is to process buffers.

By default calls handle_process/4 for each buffer.

For pads in pull mode it is called when buffers have been demanded (by returning :demand action from any callback).

For pads in push mode it is invoked when buffers arrive.