Reassigning to Mark to document edge cases and determine whether it should be in the regular documentation or the spec, since in theory it should not be visible to the consumer since it is not in the production data, although it appears in the GitHub data from which the production data is generated.
Currently people are searching the spec for what this means but it is not currently described.
Triple-up-arrow is sparsely documented. It should be noted in TR35 somewhere. I only found it in a blog post and in
https://cldr.unicode.org/downloads/cldr-36#h.6fxhmjlanm8e_l