10 Hz Resolution Bandwidth (RBW) is a software enhancement to the IF DSP of the DA851. We all have compatible hardware for it, but Rigol China is NOT going to make it available to us. So the ONLY way to get it is to figure out a hack for it. I have the 10 Hz RBW software on my unit (but it is about to expire), and I understand that there are quite a few others out there that also have it, but just haven't noticed it (?). I kind of wish that I had never seen it, because now I want it permanently. But it is being saved for the next generation of the DSA800.
The 10 Hz RBW works great and it gives you 10 dB more on sensitivity for seeing the weak signals down in the noise. When measuring IMD of a SSB transmitter the resolution between the two signal and their IMD product is much cleaner, and you can see IMD products that you can't with the 100 HZ RBW. If you are testing for IMD products of -35 dB or better you should be using 10 Hz RBW.
Rigol advertises and spec's that you can differentiate two signal 100 Hz apart. Well this is pushing the reality a bit, but with the 10 Hz RBW it is very easy and clear.