Add validation for IdentityAttribute tags

_docs_integrate/attribute-introduction.md

@@ -32,7 +32,10 @@ This guide provides an introduction to [Attributes]({% link _docs_integrate/data
 
 ### IdentityAttribute value types
 
-The [IdentityAttribute value types]({% link _docs_integrate/attribute-values.md %}#identity-attributes) are used to define the type of the `value` property of an [IdentityAttribute]({% link _docs_integrate/data-model-overview.md %}#identityattribute). In order to fulfill various purposes, many such IdentityAttribute value types are provided. If the IdentityAttribute value type used is not sufficient to clearly characterize the IdentityAttribute, `tags` can be used for a more precise description of its meaning.
+The [IdentityAttribute value types]({% link _docs_integrate/attribute-values.md %}#identity-attributes) are used to define the type of the `value` property of an [IdentityAttribute]({% link _docs_integrate/data-model-overview.md %}#identityattribute).
+In order to fulfill various purposes, many such IdentityAttribute value types are provided.
+If the IdentityAttribute value type used is not sufficient to clearly characterize the IdentityAttribute, `tags` can be used for a more precise description of its meaning.
+Apart from custom `tags`, which must have the prefix `x+%+` or `X+%+`, only Backbone-defined `tags` may be used, which can be queried using the [Get AttributeTagCollection]({% link _docs_use-cases/use-case-consumption-get-attributetagcollection.md %}) use case.
 
 The storage of multiple IdentityAttributes of the same value type is possible. If an Identity receives a [Request for an IdentityAttribute of a certain value type]({% link _docs_integrate/read-attributes-from-peer.md %}#example-of-reading-an-identityattribute), it can choose which of the matching IdentityAttributes it wants to provide for its [Response]({% link _docs_integrate/data-model-overview.md %}#response) to the [Request]({% link _docs_integrate/data-model-overview.md %}#request). For example, storing multiple IdentityAttributes of the same value type is beneficial if an Identity has more than one residential address. Using `tags` helps to distinguish such IdentityAttributes from each other.
 {: .notice--info}