Credo.CLI.Filename.pos_suffix

You're seeing just the function pos_suffix, go back to Credo.CLI.Filename module for more information.
Link to this function

pos_suffix(line_no, column)

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Returns a suffix for a filename, which contains a line and column value.

iex> Credo.CLI.Filename.pos_suffix(39, 8)
":39:8"

iex> Credo.CLI.Filename.pos_suffix(39, nil)
":39"

These are used in this way: lib/credo/sources.ex:39:8