You're seeing just the function request_line, go back to Circuits.GPIO.Chip module for more information.
Link to this function

request_line(chip, offset, direction)

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request_line(t() | String.t(), offset(), line_direction()) ::
  {:ok, Circuits.GPIO.Chip.LineHandle.t()}

Request a line handle for a single GPIO offset

{:ok, line_handle} = Circuits.GPIO.Chip.request_line(my_gpio_chip, 17, :output)

See Circuits.GPIO.Chip.request_lines/3 and Circuits.GPIO.LineHandle for more details about line handles.