Originally, prior to OpenMRS 1.7, concept_name_tag was used to indicate a concept’s preferred name, and preferred name in locales.
But this changed significantly in OpenMRS 1.7: those use cases are now handled by the concept_name_type and locale_preferred columns in the concept_name table (as you noticed).
(Which is the example you found? If it’s on the wiki, and stale, we should correct it.)
In modern OpenMRS, you would use concept_name_tag when you want to mark a specific concept name to be used in a specific context without (a) marking it as the globally-preferred name_ (which would then be used everywhere in the UI), or (b) marking or as a short name (e.g. the built-in special case suitable for use in reports, tables, etc).
I have personally used concept_name_tag just one time. In the Ebola module we had an “Ebola Preferred” concept name tag, and in our custom touch-screen UI for use in the Ebola wards, we displayed the names with this tag.
This is the code it took to do this: https://github.com/search?q=ebola+conceptnametag&ref=searchresults&type=Code&utf8=✓
One thing you’ll notice is that there is no support for concept_name_tag in the OpenMRS UI. To use them you need to (a) set up the tags via code, or directly in the DB, (b) have a custom UI that displays concepts based on the tag.
PS- This is more info than you need but: the underlying reason we needed to do this in the Ebola module is that in a few cases we wanted the preferred name in the UI to be a name that was in the CIEL dictionary as a short name. But OpenMRS doesn’t allow a short name to be marked as preferred, so in order to (a) use the CIEL dictionary, and (b) have “mg” be the preferred display name instead of “milligram”, we needed to use a concept_name_tag to mark “mg”, and use this tag rather than the usual preferred behavior.