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 hot swap backplane daemon, which both reacts to the hot swap being detected, and
 also creates detection records of what drives are present.
 
+### Associations
+
+Entity Manager will automatically create associations between its entities in
+certain cases. For details see [here](docs/associations.md).
+
 ## Requirements
 
 1. Entity manager shall support the dynamic discovery of hardware at runtime,
diff --git a/docs/associations.md b/docs/associations.md
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+# Associations
+
+Entity Manager will create [associations][1] between entities in certain cases.
+
+## `contained_by`, `containing`
+
+Entity Manager can model the [physical topology][2] of how entities plug into
+each other when upstream and downstream ports are added as `Exposes` elements.
+It will then create the 'upstream containing downstream' and 'downstream
+contained_by upstream' associations for the connected entities.
+
+For example, taken from the referenced physical topology design:
+
+superchassis.json:
+
+```json
+{
+  "Exposes": [
+    {
+      "Name": "MyPort",
+      "Type": "BackplanePort"
+    }
+  ],
+  "Name": "Superchassis"
+}
+```
+
+subchassis.json:
+
+```json
+{
+  "Exposes": [
+    {
+      "ConnectsToType": "BackplanePort",
+      "Name": "MyDownstreamPort",
+      "Type": "DownstreamPort"
+    }
+  ],
+  "Name": "Subchassis"
+}
+```
+
+Entity Manager will create the 'Superchassis containing Subchassis' and
+'Subchassis contained_by Superchassis` associations, putting the associations
+definition interface on the downstream entity.
+
+## `powered_by`, `powering`
+
+In addition to the `containing` associations, entity-manager will add
+`powering`/`powered_by` associations between a power supply and its parent when
+its downstream port is marked as a `PowerPort`:
+
+```json
+{
+  "ConnectsToType": "Mobo Upstream Port",
+  "Name": "PSU $BUS Downstream Port",
+  "Type": "DownstreamPort",
+  "PowerPort": true
+}
+```
+
+[1]:
+  https://github.com/openbmc/docs/blob/master/architecture/object-mapper.md#associations
+[2]: https://github.com/openbmc/docs/blob/master/designs/physical-topology.md