Updated Design: Subflow Instance properties (markdown)

Design:-Subflow-Instance-properties.md

@@ -10,4 +10,15 @@ This design needs to cover:
  - how the nodes within the subflow can use the customised properties
  - how an subflow instance node provides custom values to those properities
 
-This will also feed into [[Design: Subflow Node modules]].
+This will also feed into [[Design: Subflow Node modules]].
+
+---
+
+*Following a [post on the forum](https://discourse.nodered.org/t/pr-subflow-instance-properties/2853) here's a copy of my response there. These points need expanding out in the design.*
+
+
+ - Subflow instance properties are evaluated when the instance is created in the runtime. They **cannot** be set by passed in messages. They **cannot** change without the subflow instance being redeployed. This is because they are used to configure nodes within the subflow - and nodes cannot have their configuration dynamically changed at runtime. For example, the address of the MQTT Broker to connect to.
+ - This means the instance properties can only be set by any of the *static* types in the TypedInput list of supported types - including environment variables. That excludes `msg`, `flow` and `global`.
+ - Subflow instance properties appear as environment variables to the nodes within the subflow. This means, as you can today, a node's property can be set to the value `${FOO}` for it to be substituted with the value of `FOO`.
+ - We have already added in git (#dev branch) the ability to get env var values in a Function node - using `env.get("FOO")`. That will allow Function nodes to programmatically access subflow instance properties.
+ - A Change node (using the Env Var TypedInput type) can be used to attach any of these properties to a `msg` object passing through the flow if that is required.