Lady, EVOLVED 
reminder that Unicode reserves certain private use tags for their own purposes, so you should avoid using those ones yourself if possible
@Lady genuine question, in what scenario would someone end up using these if they’re not defined/widely used/documented tags?
@james use the unicode tags? mostly you wouldn’t; most of them aren’t even defined, but there are a few. Here are some examples :—
• Script code Qaag is currently being used for identifying nonstandard use of Burmese characters for the Zawgyi font, ideally to aid in migration to a standardized form. See <https://en.wikipedia.org/wiki/Zawgyi_font#Unicode_incompatibility_(ad_hoc_font_encodings)>.
• Region QO is used to mean “Outlying Oceania”
generally speaking, you would not use these tags yourself, unless you are working in the sort of narrow field which requires them. the bigger issue is if you are developing your OWN private‐use tags for some purpose (for example, to tag a constructed language in a fantasy novel). in that case, it would be ill‐advised to use a tag that Unicode CLDR has reserved for their own purposes, because Unicode CLDR is ubiquitous in computers and it might acquire some weird edge‐case meaning later.
this is documented almost nowhere so if you don’t spend your days looking thru highly technical character set algorithms and documentation you would be forgiven for not knowing