Merge pull request #552 from microsoft/corranrogue9/enumnaming

Add guidance for naming flags vs non-flags enums and cases where they should be defined side-by-side

Author: mikepizzo

graph/GuidelinesGraph.md

@@ -99,7 +99,9 @@ Following is a short summary of the most often used conventions.
 | :no_entry: **MUST NOT** use redundant words in names.                   | - **Right:** /places/{id}/**displayName** or /phones/{id}/**number** <BR> -  **Wrong:** /places/{id}/**placeName** or /phones/{id}/**phoneNumber** |
 | :warning: **SHOULD NOT** use brand names in type or property names.     | - **Right:** chat   <BR> -  **Wrong:** teamsChat <BR> - **NOTE:** there is an exception for resources that *only* exist under the `/admin` root segment and the `/users/{userId}/settings` path.  |
 | :warning: **SHOULD NOT** use acronyms or abbreviations unless they are broadly understood. | - **Right:** url or htmlSignature <BR> - **Wrong:** msodsUrl or dlp |
-| :heavy_check_mark: **MUST** use singular nouns for type names.          | - **Right:** address  <BR> - **Wrong:** addresses  |
+| :heavy_check_mark: **MUST** use singular nouns for non-enum type names.          | - **Right:** address  <BR> - **Wrong:** addresses  |
+| :heavy_check_mark: **MUST** use singular nouns for non-flags enum type names.          | - **Right:** color  <BR> - **Wrong:** colors  |
+| :heavy_check_mark: **MUST** use plural nouns for flags enum type names.          | - **Right:** diplayMethods  <BR> - **Wrong:** displayMethod  |
 | :heavy_check_mark: **MUST** use plural nouns for collections (for listing type or collection properties). | - **Right:** addresses <BR> - **Wrong:** address |
 | :ballot_box_with_check: **SHOULD** pluralize the noun even when followed by an adjective (a *postpositive*).| - **Right:** passersby or mothersInLaw    <BR> -  **Wrong:** notaryPublics or motherInLaws |
 | **CASING** | |

graph/patterns/enums.md

@@ -131,3 +131,13 @@ With such enum, customers can select multiple values in a single field:
 `displayMethod = tip | alert`
 
 In cases where two properties want to use the same *conceptual* `EnumType`, but one property is a collection while the other is single-values, the model should define *two* separate `EnumType`s, one being a non-flags enum with a singular name and the other marked as a flags enum with its name being the plural form of the non-flags enum.
+
+#### Flag enum + non-flag enum
+
+There are occasions where one API will want to use a non-flag enum, but another API will want a flags enum. 
+For example, the `displayMethod` example above may have one API that is configuring which display methods to use, and another API which is configuring that particular display method. 
+In this case, the first API will want a flags enum, but the second API will want to only allow configuring one display method at a time, and will therefore prefer a non-flags enum.
+
+Two enum types should be defined, one as a flags enum and the other as a non-flags enum.
+The flags enum should be named such that it is plural, and the non-flags enum should be named such that it is singular.
+The two types should be kept in sync with each other.