When looking for these test questions, try looking in the index. Look for key words and you'd be amazed at what you can find. Or a quick look in the contents can help you also. This is special equipment, so look in Chapter 6, Article 647.
With respect to what? Generally sensitive equipment is sensitive to the voltage from neutral to ground, and harmonics generated by the equipment itself and others shrared on the same circuit.
There are several techniques that can be used to serve sensitive equipment such as:
1. Dedicated circuits with conductors to limit the voltage drop between neutral and ground below 2 volts.
2. IG outlets to prevent circulating ground currents.
3. Balanced 120/60 volts systems that do not use a neutral. These take special transformers and distribution systems that are covered in 647. There are several restrictions using this method.
4. External signal ground networks in addition to ECG requirements.
To answer your question 0 volts between any ground.