HP 3478A Quick Reference Manual page 3

There are 5 connectors used for measurements. The middle connection on the right is the
normal common connection for voltage and current readings. The upper connecton on
the right is the normal positive connection for ac and dc readings. The lower connection
on the right side is for current readings. This is the positive connection for all mA and A
readings, with the middle (LO) connection being negative.
Resistance readings take two forms, two wire and 4 wire (Kelvin) readings. For normal
resistance readings with two probes, use the 2 WIRE Ω mode and the two upper right
hand connections. 4 WIRE Ω mode and the two left hand connections should only be
used when Kelvin clips are installed.