Jenkins launches only one build when multiple upstreams triggered the same project at the same time. This field specifies from which upstream build to copy artifacts in those cases. "Use the oldest" copies artifacts from the upstream build with the smallest build number (that is, oldest). "Use the newest" copies artifacts from the upstream build with the largest build number (that is, newest). The default value is "Use global setting", which behaves as configured in "Manage Jenkins" > "Configure System".