||Another mouthful"Two Tone Second-Order Intermodulation Distortion Dynamic Range". But this test is not hard to understand, if you take it one step at a time.
Two or more strong signals outside a receiver's passband can generate false signals (called "IMD products").
In the case of second-order IMD products, the IMD products appear at the sum or the difference of the problematic signals (see the graphic below).
We connect two signal generators to the receiver's antenna connector (using a hybrid combiner and attenuator to prevent the signal generators from interfering with each other). Then we connect a true RMS voltmeter to the receiver's AF output.
We test for IMD products on 14 MHz (or 7 MHz in the case of a single band, 40 meter rig). We tune the receiver to 14.020. One signal generator is set to 6.000 MHz and the other to 8.020 MHz. Then we crank up the input from the signal generators (by reducing attenuation) until we get a 3 dB increase on the AF meter.
The difference between the net power being delivered by the signal generators and the MDS is the "Two Tone Second-Order IMD Dynamic Range." The higher the better.
For many years, ARRL did not test for second-order IMD products. Then, in 1994, Ulrich Rohde stirred up interest in second-order IMD in his multi-part article "Key Components of Modern Receiver Design," published in QST. Since then, ARRL has paid more attention to second-order IMD in its product review. Oddly, the reports in QST include intercept points for second-order IMD, but do not include second-order dynamic range (although you can calculate the dynamic range, and you can also find the dynamic range figures in the "Expanded Reports" published in the ARRL's web site, for members only).
Theoretically, it is much easier to deal with second-order products than third-order. Third-order problematic signals can be very close to a receiver's passband, making it impossible to filter them with the receiver's front end. But second-order problematic signals are far removed from the passband, which should make them easy to filter.
Guess what? Many receivers these days have primitive filtering in their front ends. Furthermore, there are a ton of huge shortwage signals just waiting to cause second-order problems. So second-order testing turns out to be important after all.