diff --git a/source/packages/edutap_wallet_google/explanation.md b/source/packages/edutap_wallet_google/explanation.md
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@@ -1,5 +1,48 @@
 # Explanations
 
-The Google Wallet API is not self explanatory.
-Here we try to give an overview how it works and how the different parts are connected.
+## Google Wallet API
+
+The [Google Wallet](https://developers.google.com/wallet/) API is not self explanatory.
+
+The Python API client follows the models and data-structure defined by Google.
+
+- Overall Google defines Wallet Classes which are templates for Wallet Objects, while latter are the actual passes, referencing it's Wallet Class.
+- There are different types of Wallet Classes and Wallet Objects, while each type is always a pair of Wallet Class and Wallet Object, i.e. `RetailClass` and `RetailObject`.
+  Those are the top-level models.
+  Additional, there are supporting top-level objects like `Issuer`, `Permissions`, and more.
+- Each Wallet Class or Wallet Object has a set of fields which are defined by Google and can not be changed.
+- Each fields type is either a simple value or another data-type with its own fields.
+
+In this client package all data-structure defined by Google are represented by Pydantic models.
+The models are defined in the `edutap.wallet_google.models` module and are documented in the [Reference](reference.md).
+
+All top-level models are registered in the `edutap.wallet_google.registry` module.
+In further code, the models are referenced by their registered name, which is the name of the class (in CamelCase).
+
+The API functions are defined in the `edutap.wallet_google.api` module.
+They follow a CRUD API, while there is no delete at Google Wallet, but a disable.
+Additional a message can be sent to a Wallet Class or Wallet Object.
+Also, a download link can be created.
+All are documented in the [Reference](reference.md) section.
+
+Most API function do take a name of a model as first parameter.
+The name is the registered name.
+Depending on the registered name used, a function might not be able to execute.
+This is checked by the API function based on the registry record of the model, where the capabilities are stored.
+The constraints are coming from the Google API and are mirrored here.
+
+
+## Contributing
+
+The sources are in a Git version control system with its main branches at [GitHub](https://github.com/edutap-eu/edutap.wallet_google).
+
+We'd be happy to see many issues, forks, and pull-requests to make this package even better.
+
+Please report any issues at our [issue tracker](https://github.com/edutap-eu/edutap.wallet_google/issues).
+
+
+## License
+
+The code is under [European Union Public Licence v1.2](https://joinup.ec.europa.eu/collection/eupl/eupl-text-eupl-12) (EUPL).
+The EUPL is an [OpenSource Initiative (OSI) approved Free and OpenSource license](https://opensource.org/license/eupl-1-2/).