The identifier_validity property on the XRF record does not appear to require a lookup table

[jasrys]

It appears from the validation rules that the only valid values for this record are “Y”, “N”, or “U” which could be handled with a boolean (i.e. true, false, or null for the “unknown” case). If we switch it to a boolean, we should remove the mandatory requirement to allow for the null/unknown case. We could also consider changing this name of this property to something like identifier_is_valid to be clearer what true/false/null mean in that context