Peanuts are actually not nuts at all, but legumes. Peanuts grow underground while tree nuts grow on trees. There are different proteins in peanut than in tree nuts that cause allergic reactions. The Nima Peanut Sensor will only detect proteins in peanut - not from any tree nuts.
Tree nuts include, but are not limited to, almonds, Brazil nuts, cashews, chestnuts, coconuts,hazelnuts, macadamia nuts, pecans, pistachios, pine nuts, and walnuts. You can read more about peanut allergies, including how they differ from tree nuts, here.