diff --git a/docs/README.md b/docs/README.md
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+++ b/docs/README.md
@@ -22,3 +22,4 @@ Documentation for specific topics can be found in the following files:
 * [Google Maps](google-maps.md)
 * [Content Security Policy (CSP)](content-security-policy.md)
 * [Search filters](search-filters.md)
+* [Extended data](extended-data.md)
diff --git a/docs/extended-data.md b/docs/extended-data.md
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@@ -0,0 +1,37 @@
+# Extended Data
+
+_Extended data_ is arbitrary information that can be attached to certain resources in the Flex API.
+Extended data values can be of any valid JSON data type, including JSON data structures like object
+and array. The purpose of extended data is to expand the ways in which the Flex API can be used.
+
+## Types of extended data
+
+Extended data is divided into two types: _public data_ and _protected data_.
+
+### Public data
+
+Public data can be written by anyone who has write permission to a resource and it is visible to
+anyone who can read that resource.
+
+### Protected data
+
+Protected data, just like public data, can be modified by anyone with write access to the resource.
+However, it can only be read by marketplace operators. It is also possible to reveal the protected
+data at a specific point in the booking transaction process, for example if listing's address is
+presented to the customer after a booking has been confirmed.
+
+## Data schema and searched
+
+Extended data works out of the box without any prior configuration to the marketplace.  Public and
+protected data can be added to listings just by passing it in the create/update request to the API.
+However, optionally a _data schema_ can be defined for an extended data key. The data schema allows
+that extended data field to be used as a search parameter in some API endpoints, like in the
+listings query endpoint.
+
+## Extended data in Flex template app
+
+The Flex web template uses extended data by default with listing and user resources. In case of the
+listings, category and amenities fields are stored in the `publicData` field. For the
+users, phone number is stored in the `protectedData` field. As for indexed data, the template app
+relies on data schemas for categories and amenities to be configured for the marketplace in the API
+as those values are used as filters in the listing search.