When a quality gate has been missed, this property determines whether the result of the associated step or the overall build will be marked as unstable or failure. The following enum values are possible for freestyle jobs:

UNSTABLE
  Set the step and build status to unstable if the quality gate has been missed.
FAILURE
  Fail the step and build if the quality gate has been missed.

For Pipelines two additional fine-grained options are available, that allow setting the status of the step without touching the overall build status:

NOTE
  Set the step to unstable if the quality gate has been missed.
ERROR
  Fail the step if the quality gate has been missed.