diff --git a/spec/registry.dtd b/spec/registry.dtd
index 312bd4008..706c651d0 100644
--- a/spec/registry.dtd
+++ b/spec/registry.dtd
@@ -18,7 +18,7 @@
     position       (open|close|standalone)  "standalone"
 >
 
-<!ELEMENT matchSignature (input?,option*,match*,override*)>
+<!ELEMENT matchSignature (input?,option*,override*,when*,matches?)>
 
 <!ELEMENT input EMPTY>
 <!ATTLIST input
@@ -37,11 +37,22 @@
     readonly       (true|false)  "false"
 >
 
+<!ELEMENT matches (match*)>
+<!ATTLIST matches
+    href           CDATA  #IMPLIED
+    validationRule IDREF  #IMPLIED
+>
+
 <!ELEMENT match EMPTY>
 <!ATTLIST match
     locales        NMTOKENS  #IMPLIED
     values         NMTOKENS  #IMPLIED
-    validationRule IDREF     #IMPLIED
+>
+
+<!ELEMENT when (when*,matches?)>
+<!ATTLIST when
+    option         NMTOKEN   #REQUIRED
+    values         NMTOKENS  #REQUIRED
 >
 
 <!ELEMENT override (input?,option*)>
diff --git a/spec/registry.md b/spec/registry.md
index e191b052f..678ba6a6a 100644
--- a/spec/registry.md
+++ b/spec/registry.md
@@ -66,8 +66,33 @@ the corresponding input and options rules.
 If multiple `<override>` elements would match the current locale,
 only the first one is used.
 
-Matching-function signatures additionally include one or more `<match>` elements
-to define the keys against which they can match when used as selectors.
+### Variant Key Matches
+
+Selector signatures can include `<matches>` and `<when>` elements
+defining the variant keys matched by the selector.
+
+Each `<matches>` contains either a (possibly empty) set of `<match>` elements or an `href` attribute.
+If `<matches>` contains both an `href` attribute and child elements
+the `href` attribute is ignored.
+If `<matches>` contains only an `href` attribute, any `validationRule` attributes are ignored. 
+The `href` attribute MUST resolve to an XML document whose root element is `<matches>`.
+The contents of the resolved XML document replaces the current `matches` element for all later processing.
+
+The `<match>` element whose `locales` best matches the current locale
+using resource item [lookup](https://unicode.org/reports/tr35/#Lookup) from LDML is used.
+An element with no `locales` attribute is the default
+(and is considered equivalent to the `root` locale).
+
+In situations where the available keys depend on option values,
+`<when>` elements can be used to select an appropriate `<matches>` element for selection.
+If the resolved or default value of a selector option
+corresponding to the `<when>` `option` attribute
+is included in its list of `values`,
+its contents are considered before any and all later `<when>` and `<matches>` elements.
+If a `<matches>` element within a matching `<when>` has a `<match>` for the current locale,
+later `<matches>` outside that `<when>` are not considered.
+
+### Function Aliases
 
 Functions may also include `<alias>` definitions,
 which provide shorthands for commonly used option baskets.
@@ -104,15 +129,25 @@ For the sake of brevity, only `locales="en"` is considered.
 
         <matchSignature>
             <input validationRule="anyNumber"/>
-            <option name="type" values="cardinal ordinal"/>
+            <option name="select" values="plural ordinal" default="plural"/>
             <option name="minimumIntegerDigits" validationRule="positiveInteger"/>
             <option name="minimumFractionDigits" validationRule="positiveInteger"/>
             <option name="maximumFractionDigits" validationRule="positiveInteger"/>
             <option name="minimumSignificantDigits" validationRule="positiveInteger"/>
             <option name="maximumSignificantDigits" validationRule="positiveInteger"/>
-            <!-- Since this applies to both cardinal and ordinal, all plural options are valid. -->
-            <match locales="en" values="one two few other" validationRule="anyNumber"/>
-            <match values="zero one two few many other" validationRule="anyNumber"/>
+            <when option="select" values="plural">
+                <matches validationRule="anyNumber">
+                    <match locales="en" values="one other"/>
+                </matches>
+            </when>
+            <when option="select" values="ordinal">
+                <matches validationRule="anyNumber">
+                    <match locales="en" values="one two few other"/>
+                </matches>
+            </when>
+            <matches validationRule="anyNumber">
+                <match values="zero one two few many other"/>
+            </matches>
         </matchSignature>
 
         <formatSignature>
@@ -144,18 +179,18 @@ Given the above description, the `:number` function is defined to work both in a
 ```
 
 Furthermore,
-`:number`'s `<matchSignature>` contains two `<match>` elements
+`:number`'s `<matchSignature>` contains multiple `<matches>` and `<when>` elements
 which allow the validation of variant keys.
-The element whose `locales` best matches the current locale
-using resource item [lookup](https://unicode.org/reports/tr35/#Lookup) from LDML is used.
-An element with no `locales` attribute is the default
-(and is considered equivalent to the `root` locale).
 
-- `<match locales="en" values="one two few other" .../>` can be used in locales like `en` and `en-GB`
-  to validate the `when other` variant by verifying that the `other` key is present
-  in the list of enumarated values: `one other`.
-- `<match ... validationRule="anyNumber"/>` can be used to valide the `when 1` variant
-  by testing the `1` key against the `anyNumber` regular expression defined in the registry file.
+- `<when option="select" values="plural"><matches><match locales="en" values="one other" ... />`
+  can be used in locales like `en` and `en-GB` if the selection type is known to be plural
+  to validate that only `one`, `other` or numeric keys are used for variants.
+- `<when option="select" values="ordinal"><matches><match locales="en" values="one two few other" ... />`
+  can be used in locales like `en` and `en-GB` if the selection type is known to be ordinal
+  to validate that only `one`, `two`, `few`, `other` or numeric keys are used for variants.
+- `<matches validationRule="anyNumber"><match values="zero one two few many other"/>` can be used
+  for all locales and selection types, validating that variant keys are either numeric
+  or use one of the plural category identifiers.
 
 ---