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+.. This work is a derivative of https://wiki.onap.org/display/DW/Modeling+Concepts#Concepts-198012600 by IBM 
+.. used under Creative Commons Attribution 4.0 International License.
+.. http://creativecommons.org/licenses/by/4.0
+.. Copyright (C) 2020 Deutsche Telekom AG.
+
+
+.. _expression:
+
+Expression
+-------------------------------------
+
+TOSCA provides for a set of functions to reference elements within the template or to retrieve runtime values.
+
+**Below is a list of supported expressions**
+
+.. tabs::
+
+   .. tab:: get_input
+
+      **get_input**
+
+      The **get_input** function is used to retrieve the values of properties declared 
+      within the inputs section of a TOSCA Service Template.
+
+      Within CDS, this is mainly Workflow inputs.
+
+      `TOSCA specification 
+      <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454178>`_
+
+      **Example:**
+
+      `<https://github.com/onap/ccsdk-cds/blob/master/components/model-catalog/blueprint-model/test-blueprint/golden/Definitions/golden-blueprint.json#L210>`_
+
+      .. code-block:: JSON
+         
+         "resolution-key": {
+         "get_input": "resolution-key"
+         }
+         
+   .. tab:: get_property
+
+      **get_property**
+
+      The **get_property** function is used to retrieve property values between modelable 
+      entities defined in the same service template.
+
+      `TOSCA specification 
+      <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454178>`_
+
+      **Example:**
+
+      TBD
+
+   .. tab:: get_attribute
+
+      **get_attribute**
+
+      The **get_attribute** function is used to retrieve the values of named attributes declared 
+      by the referenced node or relationship template name.
+
+      `TOSCA specification 
+      <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454178>`_
+
+      **Example:**
+
+      `<https://github.com/onap/ccsdk-cds/blob/master/components/model-catalog/blueprint-model/test-blueprint/golden/Definitions/golden-blueprint.json#L64-L67>`_
+
+      .. code-block:: JSON
+         
+         "get_attribute": [
+            "resource-assignment",
+            "assignment-params"
+         ]
+
+   .. tab:: get_operation_output
+
+      **get_operation_output**
+
+      The **get_operation_output** function is used to retrieve property values between modelable 
+      entities defined in the same service template.
+
+      `TOSCA specification 
+      <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454180>`_
+
+      **Example:**
+
+      TBD
+
+   .. tab:: get_artifact
+
+      **get_artifact**
+
+      The **get_artifact** function is used to retrieve property values between modelable 
+      entities defined in the same service template.
+
+      `TOSCA specification 
+      <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454182>`_
+
+      **Example:**
+
+      TBD
+\ No newline at end of file