You're seeing just the macro def_input_pad, go back to Membrane.Bin module for more information.
Link to this macro

def_input_pad(name, spec)

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Macro that defines input pad for the bin.

Allows to use one_of/1 and range/2 functions from Membrane.Caps.Matcher without module prefix.

It automatically generates documentation from the given definition and adds compile-time caps specs validation.

The type Membrane.Pad.bin_spec_t/0 describes how the definition of pads should look.