CLDR-17893 Clarify rg validation for unit preferences (#4049)

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

docs/ldml/tr35-info.md

@@ -1235,10 +1235,20 @@ If any key-values are invalid, then they are ignored. Thus the following constru
 | --- | --- |
 | -mu-smoot | invalid unit |
 | -ms-stanford | invalid unit system |
-| -rg-aazzzz | invalid region 'AA' ‡|
-| -AA | invalid region 'AA'|
+| -rg-abzzzz | invalid region 'AB' ‡|
+| -AB | invalid region 'AB'|
 
-‡ Only the region portion is currently used, so in -rg-usabcdef the "abcdef" is ignored, whether or not it is valid.
+‡ Only the region portion is currently used.
+The -rg-abzzzz is ignored because AB is invalid; 
+if it were -rg-ustuvxy, it would not be ignored because US is valid.
+The table below shows when the region portion is valid or not.
+
+| Key-value | Region | Valid? | Comment |
+| --- | --- | --- | --- |
+| -rg-usut | US | Yes | Both the region portion (US) and the subdivision portion (ut = Utah) are valid. |
+| -rg-uszzzz | US | Yes | Both the region portion (US) and the subdivision portion (zzzz = all) are valid. |
+| -rg-usabc | US | Yes | The region portion (US) is valid, but the subdivision portion (abc) is not. |
+| -rg-abzzzz | AB | No, ignored | The region portion (AB) is invalid, and thus the -rg is ignored, not matter that the subdivision portion (zzzz) is. |
 
 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 +1263,10 @@ 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 portion of the rg value, let R be that region, and go to Compute the category.
+	* See the table above for the examples `usut`, `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