Unique item IDS per collection

CHANGELOG.md

@@ -38,6 +38,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 - Several typos and minor language changes
 - Clarified that collection IDs should be unique across all collections in the corresponding root catalog
+- Clarified that item IDs should be unique per collection
 - Clarified which media types should be used for the hierarchical relation types
 - Clarified in the Markdown specification that GeometryCollections are not allowed as Item Geometry ([#1160](https://github.com/radiantearth/stac-spec/pull/1160))
 - Best practice: Do not recommend subdirectories for Items without sidecar files ([#1195](https://github.com/radiantearth/stac-spec/pull/1195))

best-practices.md

@@ -182,9 +182,10 @@ The relative URLs `folder/item.json` and `./folder/item.json` are equivalent.
 
 When defining one's STAC properties and fields there are many choices to make on how to name various aspects of one's
 data. One of the key properties is the ID. The specification is quite flexible on ID's, primarily so that existing
-providers can easily use their same ID when they translate their data into STAC - they just need to be sure it is globally
-unique, so may need a prefix. But the use of URI or file path reserved characters such as `:` or `/` is discouraged since this will 
-result in [percented encoded](https://tools.ietf.org/html/rfc3986#section-2) [STAC API](https://github.com/radiantearth/stac-api-spec) 
+providers can easily use their same ID when they translate their data into STAC.
+It is STRONGLY RECOMMENDED that an item ID is unique per collection.
+The use of URI or file path reserved characters such as `:` or `/` is discouraged since this will
+result in [percented encoded](https://tools.ietf.org/html/rfc3986#section-2) [STAC API](https://github.com/radiantearth/stac-api-spec)
 endpoints and it prevents the use of IDs as file names as recommended in the [catalog layout](#catalog-layout) best practices.
 
 ### Searchable Identifiers

item-spec/item-spec.md

@@ -58,7 +58,7 @@ inherited from GeoJSON.
 | type            | string                                     | **REQUIRED.** Type of the GeoJSON Object. MUST be set to `Feature`.                                                                                                                                                                                                                                       |
 | stac_version    | string                                     | **REQUIRED.** The STAC version the Item implements.                                                                                                                                                                                                                                                       |
 | stac_extensions | \[string]                                  | A list of extensions the Item implements.                                                                                                                                                                                                                                                                 |
-| id              | string                                     | **REQUIRED.** Provider identifier. The ID should be unique within the  [Collection](../collection-spec/collection-spec.md) that contains the Item.                                                                                                                                                        |
+| id              | string                                     | **REQUIRED.** Provider identifier. The ID should be unique within the [Collection](../collection-spec/collection-spec.md) that contains the Item.                                                                                                                                                         |
 | geometry        | GeoJSON Geometry Object \| null            | **REQUIRED.** Defines the full footprint of the asset represented by this item, formatted according to RFC 7946, [section 3.1](https://tools.ietf.org/html/rfc7946#section-3.1) if a geometry is provided or [section 3.2](https://tools.ietf.org/html/rfc7946#section-3.2) if *no* geometry is provided. |
 | bbox            | \[number]                                  | **REQUIRED if `geometry` is not `null`, prohibited if `geometry` is `null`.** Bounding Box of the asset represented by this Item, formatted according to [RFC 7946, section 5](https://tools.ietf.org/html/rfc7946#section-5).                                                                            |
 | properties      | [Properties Object](#properties-object)    | **REQUIRED.** A dictionary of additional metadata for the Item.                                                                                                                                                                                                                                           |