CLDR-17265 Document special script codes (#3594)

Co-authored-by: Steven R. Loomis <[email protected]>

docs/ldml/tr35.md

@@ -698,6 +698,37 @@ Private use codes fall into three groups.
 
 See also _[Unknown or Invalid Identifiers](#Unknown_or_Invalid_Identifiers)_.
 
+### Special Script Codes
+Certain valid script code require special handling.
+These are the codes in [Script Codes](https://www.unicode.org/iso15924/iso15924-codes.html) with the words "variant" or "alias" within parentheses,
+excluding Zsye. 
+The Compound codes include characters in multiple scripts; 
+the Visual variants are distinct in appearance, but otherwise encompass a single script; 
+and the Subsets exclude certain characters from a script. 
+The Equivalents for Subsets are not as well defined, so the "Equivalents" are marked as approximate.
+
+| Group | Script | Equivalent |
+| --- | --- | --- |
+| Compounds | Jpan | ≡ Hani ∪ Hira ∪ Kana |
+|  | Hrkt | ≡ Hira ∪ Kana |
+|  | Kore | ≡ Hani ∪ Hang |
+|  | Hanb | ≡ Hani ∪ Bopo |
+| Visual variants | Aran | ≡ Arab (Nastaliq variant) |
+|  | Cyrs | ≡ Cyrl (Old Church Slavonic variant) |
+|  | Latf | ≡ Latn (Fraktur variant) |
+|  | Latg | ≡ Latn (Gaelic variant) |
+|  | Syrn | ≡ Syrc (Eastern variant) |
+|  | Syre | ≡ Syrc (Estrangelo variant) |
+|  | Syrj | ≡ Syrc (Western variant) |
+| Subsets (approximate) | Jamo | ≃ Hang - LVT - LV |
+|  | Hans | ≃ Hani - Traditional-only |
+|  | Hant | ≃ Hani - Simplified-only |
+
+The special codes most frequently used are in the locale identifiers zh-Hans, zh-Hant, ja-Jpan, and ko-Kore. 
+These are used, for example, in [Likely Subtags](#Likely_Subtags) in LDML. 
+Some of the special codes are used in other specifications, 
+such as in [Mixed_Script_Detection](https://unicode.org/reports/tr39/#Mixed_Script_Detection).
+
 <a name="Locale_Extension_Key_and_Type_Data"></a>
 ### <a name="u_Extension" href="#u_Extension">Unicode BCP 47 U Extension</a>