Nvidia CP, Low Latency Mode, Custom setting?

[magellan]

For the Last of Us Part 1, in the Nvidia CP, it has the Low Latency Mode set to "Custom", I've never seen this setting before and it can't be selected from the drop-down menu. Has anyone seen this before? What does the "Custom" setting imply? There's no explanation for it given and my google-fu came up w/nothing.