analysiscurrentcurrent-sourcevoltage
Suppose you have a circuit 
,you are told that the voltage at point 2 relative to point 1 is -90V, the voltage across R2 is -30V and V0 is 200V.
What is the meaning of voltage at 2 relative to 1? Does it mean the potential difference between 2 and 1 as in (voltage at point 2) – (voltage at point 1) or is it vice versa?
Does voltage at a point relative to another mean a voltage drop between those two? And what roles do the negative voltages at either points play?
(In ideal conditions) You're correct in your understanding of the first case. When you close the switch, you short one end of the resistor to ground, so there's a 12V drop across it and I=0.02A.
In your second case, with the switch closed, you are creating a voltage divider so the point between the resistors is at 6V.
If you don't have a load connected along Vout (between the black dot and ground), then I=0 there since there isn't a voltage drop along the horizontal connection.
Measuring along the path connecting the resistors/switch, you'll see your 0.01A, and since there is no other path for the current to take, both resistors will see the full 0.01A.
You won't be able to measure it with a voltmeter, but there will indeed be a potential difference. In a P-N junction with no external voltage applied negative charges from the N-doped region will migrate to the P-doped region, and vice versa. The blue region will be negative charged, the red one positive. The difference in charge causes a 0.7V across the barrier. (Will depend on the donor and receptor concentration.)
Best Answer
Voltage is always relative. When you just hold one probe of your DMM to the + of a battery, for instance it will display 0 V, like it's saying "this is nothing, what do I have to compare it with?".
Any circuit should have a ground (yours hasn't). Ground is your reference everything else is compared to. Usually that's the negative terminal of the power supply. It's only by calling ground 0 V that the supply's +12 V makes sense: it's 12 V higher than ground. If you measure 2.7 V in your circuit the "referenced to ground" is implied, unless indicated otherwise. That's the case when you say "relative to X", then you don't use ground as a reference, but the voltage at X.
For DC normal arithmetic applies: if A is +5 V relative to ground and B = +12 V relative to ground then A is -7 V relative to B.