imperfect indistinguishability

and the conditions where it might be OK

design-dimensions/Dimensions-with-General-Agreement.md

@@ -49,3 +49,6 @@ Users must be able to opt out of participation in any measurement activitity.
 
 If a user opts out, then it must be possible for their user agent to hide this fact from sites.
 For example, in an aggregation service, the user agent might provide a zero value to contribute to the aggregate, but this needs to be indistinguishable from a non-zero value.
+
+In some designs, an opt-out might not be perfectly indistinguishable, but opt out must be indistinguishable from other normal and commonplace conditions.
+For example, if opt out in measurement results in returning no information in response to a request, then participating users must also regularly return no information for reasons other than opt out.