The default rule is a list of buckets.
If a specific id did not match any rule, then this rule is applied in order to determine the treatment to be returned.

Note that you do not need to provide all treatments in the default rule, however, the sum of treatment sizes must be 100

{
  "rules" : [{ "..." }],
  "defaultRule": [ 
    { "treatment": String, size: Number },
    { "treatment": String, size: Number }, 
    … 
  ]
}

If no rule is matched for the id, then the default rule is evaluated and there is a 90% chance that the result is on, and 10% chance that it is off.