CLDR-17893 Clarify rg validation for unit preferences

docs/ldml/tr35-info.md

@@ -1239,6 +1239,13 @@ If any key-values are invalid, then they are ignored. Thus the following constru
 | -AA | invalid region 'AA'|
 
 ‡ Only the region portion is currently used, so in -rg-usabcdef the "abcdef" is ignored, whether or not it is valid.
+The table below shows when the region portion is valid or not.
+
+| Subdivision | Region | Used in -rg? | Comment |
+| --- | --- | --- | --- |
+| usca | US | Yes | Both the subdivision code (US Texas) and its region portion (US) are valid |
+| usabc | US | Yes | The subdivision code is invalid, but its region portion (US) is valid |
+| abcdef | AB | No | Both the subdivision code and its region portion (~AB~) are invalid |
 
 The following algorithm is used to compute the override units, regions, and category.
 The latter two items are used in the [Unit Preferences Data](#Unit_Preferences_Data).
@@ -1253,9 +1260,10 @@ If there is no valid -mu value, the following steps are used to determine a regi
 
 1. If there is a valid -ms value then let USM  be the corresponding value in column 2 of the table below.
 Otherwise FR is not used. In either case continue with step 2.
-2. If there is a valid -rg region, let R be that region, and go to Compute the category.
-3. If there is a valid region in the locale, let R be that region, and go to Compute the category.
-4. Otherwise, compute the likely subtags for the locale.
+2. If there is a valid -rg region code as the prefix of its subdivision value, let R be that region, and go to Compute the category.
+	* See the table above for the examples `usca`, `usabc`, and `abcdef`
+4. If there is a valid region in the locale, let R be that region, and go to Compute the category.
+5. Otherwise, compute the likely subtags for the locale.
      1. If there is a likely region, then let R be that region, and go to Compute the category.
 	 2. Otherwise, let R be 001, and go to Compute the category