Hi Laurence,

Per the ST1-4   -  Product Setup Guide.pdf:

However, this field is not normally used for identifying the activity classification.  It would also have to be populated by the user manually (unless you use a Process Flow to update the value upon a save event if it is null) so I am not sure you want to use this.

When you define an activity, you can assign an activity type. From the same document:

The activity type is a definition field populated in the Activity Codes module and can be maintained here.  This I think is more aligned with you are trying to do as that is associated directly with the activity code and does not require the user to populate the demand type in the service order document.