When validating a code, both tx.fhir.org and ontoserver return a list of issues that were found with the code. E.g. not found in the code system, not found in the value set, invalid display, etc. Each of these issues needs a 'type' so that the issue can be correctly handled in the client code - .e.g the validator will downgrade some kind of errors to warnings based on the binding strength or other considerations. 

The test cases for compliant terminology servers therefore require these issues to have a type taken from a code system defined in the tools IG: https://build.fhir.org/ig/FHIR/fhir-tools-ig/CodeSystem-tx-issue-type.html

Given its importance, we should consider moving this code system to either the core FHIR specification or THO. This will be good for handling tasks like FHIR-44810 that ak for an example of an issue that uses this code system