Power from Vin

Can I power Arduino UNO from Vin pin? What happens if I supply 5V to Vin and connect to USB at the same time?

Vin is connected to the power jack through a diode.
You need at least 7 volts at the jack to power the Arduino.
If you put power in at Vin you need at least 7-.6=6.4volts, hence, you cannot use 5volts at Vin.
You can power the Arduino at the same time with USB.

Vin is after the diode.
If Vin/2 is <3.3V, then USB power will be used.

LarryD:
Vin is connected to the power jack through a diode.
You need at least 7 volts at the jack to power the Arduino.
If you put power in at Vin you need at least 7-.6=6.4volts, hence, you cannot use 5volts at Vin.
You can power the Arduino at the same time with USB.

Is diode to protect Arduino from reverse polarity?
So I can supply 12V to Vin, right? The 12V supply is also used for powering motors in my project so I doubt there would be some interference to Arduino.

Is diode to protect Arduino from reverse polarity?
Yes
So I can supply 12V to Vin, right?
You could but I would just apply it to the input jack.
The 12V supply is also used for powering motors in my project so I doubt there would be some interference to Arduino.
If it powers motors it probably will cause noise and problems with the Arduino.

LarryD:
Is diode to protect Arduino from reverse polarity?
Yes
So I can supply 12V to Vin, right?
You could but I would just apply it to the input jack.
The 12V supply is also used for powering motors in my project so I doubt there would be some interference to Arduino.
If it powers motors it probably will cause noise and problems with the Arduino.

In fact, I was planning to give power to Arduino from BEC 5V output of this ESC. But if it's not possible, I have to choice but to supply 12V directly to Arduino.