As discussed in medium-gory detail in this paper, these devices can let you do shot-noise limited measurements at baseband with lasers that are as much as 70 dB noisier than that.
They're limited by two main effects: beta nonlinearity (1/hFE-1/hfe) and log nonconformance (d ln(IC)/dVBE - kT/e).
This tester measures both of these quantities directly. It's a one-off, of course, so it's done with discrete logic and instrumentation amp parts. It has certain points of interest, for instance the use of a unity gain instrumentation amplifier as a precision +1/+2 gain amplifier. One loose end: U1 is a LT1043 switched-capacitor building block—a glorified MUX that has very low charge injection and very good common-mode rejection.