A sensitivity question

I got two sensitivity spec from two different datasheets. One discribes the
sensitivity is 2μV, and the other one is 2μV(emf).
I have two questions here.
1) Which performance is better?
2) Why we describe the sensitivity in emf rather than uV only? what's
the reason behind this?

Thanks,

Some RF signal generators are calibrated not only in dBm or uV, but also in uV EMF (ElectroMotive Force). If a generator is loaded with impedance equal to its designed output impedance then the open circuit output voltage (EMF) will be halved.
The advantage of using EMF is that the match is not assumed.
Using only microvolts, an input impedance is assumed (usually 50 ohms, otherwise is specified).

You may like to refer to the book by trum.

Would you tell me the book title?
Thanks,