diff --git a/.github/instructions/API_conventions.instructions.md b/.github/instructions/API_conventions.instructions.md
index 6bf6e75a39de..13e542889fce 100644
--- a/.github/instructions/API_conventions.instructions.md
+++ b/.github/instructions/API_conventions.instructions.md
@@ -20,3 +20,8 @@ When helping me write code or reviewing my existing code, please follow these sp
    - Note: This is correct only when the type is the same in both TS annotation and JSDoc comment.
 
 Consistently apply these conventions to all code you review, suggest, complete.
+
+## Related instructions
+
+- Optional field typing (`?`, `| undefined`, `| null`, `@default`) for public `.d.ts`:
+  see [optional-fields-typing.instructions.md](./optional-fields-typing.instructions.md).
diff --git a/.github/instructions/optional-fields-typing.instructions.md b/.github/instructions/optional-fields-typing.instructions.md
new file mode 100644
index 000000000000..b5faf99aee94
--- /dev/null
+++ b/.github/instructions/optional-fields-typing.instructions.md
@@ -0,0 +1,109 @@
+---
+applyTo: "**/packages/devextreme/js/**/*.d.ts"
+---
+
+# Optional field typing (`?`, `| undefined`, `| null`, `@default`)
+
+When you write, complete, or review an optional field in a public `.d.ts`, apply the rules
+below. The shape of an optional field is decided by two independent axes.
+
+## Two axes
+
+- **Type axis (`| undefined`).** Add `| undefined` only if the field is an OPTION whose
+  stored default in `defaultOptions` is `undefined`. Otherwise use a bare `?`.
+- **`@default` axis (JSDoc).** Write `@default <X>` only when `X` is actually STORED as the
+  default — in `defaultOptions` for an option, or as the seed object for an object-valued
+  option. A point-of-use default (a `switch` default, a destructuring default, `??`) is NOT
+  stored: do not write `@default`. (The on-omission behavior is documented by technical
+  writers in a separate documentation repository, not in the `.d.ts`.)
+
+Across the board: in `defaultOptions`, "no value" is `undefined`, not `null`. Use `null`
+only when the runtime performs a meaningful `=== null` check.
+
+## New vs existing fields (breaking-change policy)
+
+- **New field:** the default in `defaultOptions` is `undefined`; the type and `@default`
+  follow `undefined`. `null` (as a type member or as a value) is allowed ONLY with explicit
+  justification — the runtime genuinely distinguishes it with `=== null`. Without that
+  justification, reject `null` in review. This is the decision the linter cannot make, so it
+  must be enforced here.
+- **Existing field:** never introduce a breaking change. Do not migrate a runtime `null` to
+  `undefined`, and do not remove an existing `| null`. Fix a type-vs-`@default` mismatch only
+  by WIDENING the type to match the runtime (for example, add `| null`); leave the runtime
+  default and the `@default` tag unchanged.
+
+## Categories
+
+- **A — scalar/collection option** (a key in `defaultOptions`): concrete stored default
+  -> `T`; stored `undefined` -> `T | undefined`; meaningful `null` -> `T | null`.
+  Callbacks/events are the `undefined` case: `((e) => void) | undefined`, `@default
+  undefined`, never `| null`.
+- **A-obj — object-valued option** (the value is a config the runtime merges, e.g.
+  `editing`, `paging`, `tooltip`, `dropDownOptions`): type is `T` without `| undefined`. A
+  `@default` (**R4**) is needed only when `_getDefaultOptions` stores a value that DIFFERS from
+  the referenced type's own defaults — i.e. a runtime override worth documenting. The referenced
+  type (`PopupProperties`) already documents its sub-properties' defaults, so an empty seed
+  (`{}`, no override) needs no `@default`. An INLINE object whose sub-properties carry their
+  own `@default` needs no container `@default`. (Not lint-enforced — a review judgment.) Avoid the "optional feature, off by default"
+  pattern (`fooOptions?: ... | undefined`) in new design — it overloads one option with both
+  the config and the on/off flag; prefer a separate boolean flag plus a config option
+  (`fooEnabled: boolean` + `fooOptions: NestedProperties`).
+- **B — object property** (a data-object field such as `Message`, or a config-item
+  sub-property such as `TextEditorButton`): bare `foo?: T` without `| undefined`; NO
+  `@default`. The default is a point-of-use fallback that is never stored, so reading the
+  option via `.option()` would not return it. Do not document the behavior in the `.d.ts` —
+  technical writers do that in a separate documentation repository; here you simply omit
+  `@default`.
+
+## Correct vs incorrect examples
+
+```ts
+// A — option whose stored default is undefined: the type must carry | undefined
+/** @default undefined */
+filterValues?: Array<any> | undefined;        // correct
+/** @default undefined */
+filterValues?: Array<any>;                     // incorrect: @default undefined, but no | undefined in the type
+
+// A — @default null requires null in the type
+/** @default null */
+editRowKey?: TKey | null;                      // correct
+/** @default null */
+editRowKey?: TKey;                             // incorrect: @default null, but the type allows neither null nor undefined
+
+// A-obj (reference) — @default documents a runtime override of the type's defaults
+/** @default { showTitle: false } */
+dropDownOptions?: PopupProperties;             // correct: _getDefaultOptions overrides showTitle
+dropDownOptions?: PopupProperties;             // correct: seed {} (no override) -> no @default
+// A-obj (inline) — sub-properties carry their own @default; the container needs none
+editing?: {
+  /** @default false */ allowDeleting?: boolean;
+};                                             // correct
+
+// B — a data-object / config-item field: no @default (value isn't stored), no prose in the .d.ts
+/**
+ * @docid
+ * @public
+ */
+type?: MessageType;                            // correct
+/** @default "after" */
+location?: TextEditorButtonLocation;           // incorrect: @default whose value isn't stored in defaultOptions
+```
+
+## Review checklist
+
+- **R1**: `@default null` present -> the type must include `null`.
+- **R2**: a concrete `@default` (not `null`/`undefined`) -> the type must NOT include `| undefined`.
+- **R3**: `@default undefined` present -> the type must include `| undefined`.
+- **R4**: an object-valued option -> add a `@default` only when `_getDefaultOptions` stores a value
+  that DIFFERS from the referenced type's own defaults (a runtime override); an empty seed
+  (`{}`) or an INLINE object whose sub-properties carry their own `@default` needs none. (Not
+  lint-enforced — a review check.)
+- A data-object / config-item field (not a stored option) -> no `@default`; remove any
+  `@default` whose value isn't stored in `defaultOptions` (a point-of-use default in a
+  `switch`/`case`). Behavior is documented by tech writers elsewhere, not in the `.d.ts`.
+- A new field meaning "no value" -> default to `undefined`, not `null`; flag any `null` that
+  has no `=== null` justification.
+- An existing field -> never propose migrating a runtime `null` to `undefined` or removing
+  `| null` (both are breaking changes); only widen the type.
+
+Apply these consistently to every `.d.ts` field you write, complete, or review.