"No match?" option controls how scripts are processed
The "No match?" option controls what to do when no rules for an option are matched. You find the setting in the advanced controls of each option panel (shown as 1 in the screenshot above).
The options are:
Default: choose the option marked with "Use as default option?"
First: choose the first item in the list (also chosen if no Default is specified). Elements in the rule block can be reordered by dragging:
Skip: skip this option block all together and move on to the next
Empty: stop processing this script and return an empty VAST response (or Null JSON response)
Empty VAST response
The VAST standard specifies (Page 9, Para 5) that any ad server in the response chain that does not want to respond, or has no ad to respond with, can return an empty tag:
This option is useful if you want to not serve anything if rules are not matched. We use this for things like accurate location targeting where we don't want to serve an ad to any user with inaccurate location information.
Be aware that returning an empty VAST response is an expensive processing task and also relies on the upstream ad servers being able to replace the missed ad with another so that the opportunity to serve an ad is not missed.