diff --git a/spec/data-model/README.md b/spec/data-model/README.md
index 1548a20d97..e0d187bf87 100644
--- a/spec/data-model/README.md
+++ b/spec/data-model/README.md
@@ -93,21 +93,14 @@ Each message _declaration_ is represented by a `Declaration`,
 which connects the `name` of a _variable_
 with its _expression_ `value`.
 The `name` does not include the initial `$` of the _variable_.
-
-The `name` of an `InputDeclaration` MUST be the same
-as the `name` in the `VariableRef` of its `VariableExpression` `value`.
+If the `value` has a `VariableRef` `arg` with the same `name`
+as the `Declaration`,
+it represents an _input-declaration_.
+Otherwise, it represents a _local-declaration_.
 
 ```ts
-type Declaration = InputDeclaration | LocalDeclaration;
-
-interface InputDeclaration {
-  type: "input";
-  name: string;
-  value: VariableExpression;
-}
-
-interface LocalDeclaration {
-  type: "local";
+interface Declaration {
+  type: "declaration";
   name: string;
   value: Expression;
 }
@@ -149,21 +142,11 @@ expressions or markup.
 ```ts
 type Pattern = Array<string | Expression | Markup>;
 
-type Expression =
-  | LiteralExpression
-  | VariableExpression
-  | FunctionExpression;
-
-interface LiteralExpression {
-  type: "expression";
-  arg: Literal;
-  function?: FunctionRef;
-  attributes: Attributes;
-}
+type Expression = OperandExpression | FunctionExpression;
 
-interface VariableExpression {
+interface OperandExpression {
   type: "expression";
-  arg: VariableRef;
+  arg: Literal | VariableRef;
   function?: FunctionRef;
   attributes: Attributes;
 }
diff --git a/spec/data-model/message.dtd b/spec/data-model/message.dtd
index bc51dd1590..848ebc6c8f 100644
--- a/spec/data-model/message.dtd
+++ b/spec/data-model/message.dtd
@@ -3,12 +3,8 @@
   (pattern | (selectors,variant+))
 )>
 
-<!-- In a <declaration type="input">, the <expression> MUST contain a <variable> -->
 <!ELEMENT declaration (expression)>
-<!ATTLIST declaration
-  type (input | local) #REQUIRED
-  name NMTOKEN #REQUIRED
->
+<!ATTLIST declaration name NMTOKEN #REQUIRED>
 
 <!ELEMENT selectors (variable)+>
 <!ELEMENT variant (key+,pattern)>
diff --git a/spec/data-model/message.json b/spec/data-model/message.json
index b669af4627..c06c3734f6 100644
--- a/spec/data-model/message.json
+++ b/spec/data-model/message.json
@@ -82,19 +82,10 @@
       }
     },
 
-    "input-declaration": {
+    "declaration": {
       "type": "object",
       "properties": {
-        "type": { "const": "input" },
-        "name": { "type": "string" },
-        "value": { "$ref": "#/$defs/variable-expression" }
-      },
-      "required": ["type", "name", "value"]
-    },
-    "local-declaration": {
-      "type": "object",
-      "properties": {
-        "type": { "const": "local" },
+        "type": { "const": "declaration" },
         "name": { "type": "string" },
         "value": { "$ref": "#/$defs/expression" }
       },
@@ -102,12 +93,7 @@
     },
     "declarations": {
       "type": "array",
-      "items": {
-        "oneOf": [
-          { "$ref": "#/$defs/input-declaration" },
-          { "$ref": "#/$defs/local-declaration" }
-        ]
-      }
+      "items": { "$ref": "#/$defs/declaration" }
     },
 
     "variant-key": {