`fare_media_id` for `fare_transfer_rules`: Wildcard or Explicit Enumeration?

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Jerome from [Transit](https://transitapp.com/)

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This question aims to clarify an ambiguity regarding `fare_media` linked with `fare_transfer_rules`.

Suppose we have a complete fare price description for the entire network, covering every available fare medium (i.e., all `fare_products` associated with `fare_leg_rules` have a non-null `fare_media_id`). Now, if we want to define a universally available free transfer, is it necessary to duplicate `fare_transfer_rules` and `fare_products` for each type of `fare_media`, or is it valid to use a null `fare_media_id` to indicate compatibility with all fare products?

In other words, does a null value for `fare_media_id` act as a wildcard, applying to any fare medium, or is it considered non compatible with other fare medium, requiring an explicit enumeration of transfer rules for each fare medium via fare_products?