Locate Item provides a strength indicator tool to help you zero in on a missing item.
You first select a row on a worksheet to indicate which tag should be located, then choose the Tools menu / Locate Item.
You can then press, hold the trigger down, and the strength indicator should be highest in the direction of the tag as you move the reader around.
RSSI = Received Strength Signal Indicator, which is a measure of power coming back from the tag.
The threshold setting lets you adjust the target RSSI level. Once
reached or exceeded, the reader plays a "located" sound.
There isn't an optimal threshold setting because the signal strength coming back from the tag is dependent on the reader's antenna orientation relative to the tag, the tag itself, and the materials in the environment.