Add nullable enum guidance: null vs none sentinel values

graph/patterns/enums.md

@@ -102,6 +102,72 @@ Additionally speaking, depending on the situation, a nullable enum can very like
 
 If used, `EnumType` names should be singular if the are non-flags enums, and the names should be plural if they are flags enums.
 
+
+#### Nullable enums
+
+Enum properties can be marked `Nullable="true"`, which means the property can hold `null` in addition to any defined member.
+Before making an enum property nullable, consider whether a sentinel member like `none` better communicates the intent.
+
+##### `null` vs `none`
+
+| Value | Meaning | Use when |
+|---|---|---|
+| `null` | The property has no value — it was never set or is not applicable | The absence of a value is semantically different from every defined member |
+| `none` | An explicit "nothing selected" choice within the enum's domain | "No selection" is a valid, intentional state the caller can set |
+
+> **Note:** The `unknownFutureValue` sentinel is always required as the last known member of every enum (see [evolvable enums](./evolvable-enums.md)).
+> It is unrelated to nullability and must be present regardless of whether the property is nullable or uses a `none` member.
+
+##### Prefer a `none` member over nullable
+
+In most cases, add a `none` member (value `0`) instead of making the property nullable.
+This keeps the property non-nullable, which is simpler for SDK consumers and avoids the three-way ambiguity of "is it null, is it none, or is it a real value?"
+
+```xml
+<!-- ✅ RECOMMENDED — explicit 'none' member -->
+<EnumType Name="priority">
+    <Member Name="none" Value="0"/>
+    <Member Name="low" Value="1"/>
+    <Member Name="normal" Value="2"/>
+    <Member Name="high" Value="3"/>
+    <Member Name="unknownFutureValue" Value="4"/>
+</EnumType>
+
+<Property Name="priority" Type="microsoft.graph.priority" Nullable="false"/>
+```
+
+##### When nullable is appropriate
+
+Use `Nullable="true"` on an enum property only when **all** of the following apply:
+
+1. **Absence is meaningful** — `null` represents "not set" or "not applicable," which is semantically distinct from every enum member including a hypothetical `none`.
+2. **A sentinel member would be misleading** — adding `none` would imply the caller actively chose "nothing," but the actual semantics are that the property doesn't apply to this instance.
+3. **The property is optional on creation** — the service does not assign a default value; `null` is the expected state until the caller explicitly sets one.
+
+```xml
+<!-- Acceptable — null means "not yet evaluated" which is distinct from any severity level -->
+<Property Name="severity" Type="microsoft.graph.severity" Nullable="true"/>
+```
+
+##### Anti-pattern: nullable + `none`
+
+Do **not** combine a nullable enum with a `none` member.
+This creates two ways to express "no value" and forces callers to handle both `null` and `none`, leading to inconsistency and bugs.
+
+```xml
+<!-- ❌ WRONG — ambiguous: is "no value" null or none? -->
+<EnumType Name="priority">
+    <Member Name="none" Value="0"/>
+    <Member Name="low" Value="1"/>
+    <Member Name="high" Value="2"/>
+    <Member Name="unknownFutureValue" Value="3"/>
+</EnumType>
+
+<Property Name="priority" Type="microsoft.graph.priority" Nullable="true"/>
+```
+
+Pick one: either `none` with `Nullable="false"`, or no `none` member with `Nullable="true"`.
+
 #### Flag Enums or Collection of Enums
 
 In case an enum can have multiple values at the same time the tentation is to model the property as a collection of Enums: