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@allObjects
We are talking BAT_IN on schematic, which on the board is the pin next to B15 - marked Bat - and the pad on the back side - marked + - (where a battery connector can be mounted). The markngs on the back are not very conclusive / matching with the schematic... rather confusing... On the other hand, only savvy poeple look at the schematics, regulars just look at the markings... and schematic's BAT_IN is board's marked Bat and + sign - no ambiguity on the board itself.
According to the schematics / the board routing, USB power feeds the regulator over a diode and the BAT(tery)_IN - Bat pin next to B15 - over the P FET. The regulator feeding point is connected to the 0.1" and 0.05" (V)BAT pins - marked VCC - grouped with the 0.1" and 0.05" GND and 3.3V pins.
I do not know the behavior when connecting a battery to (V)Bat:
I'm not savy about the PFET's behavior / what happens when you feed it on the (V)Bat / VCC / regulator feeding point.
The Diode protects 'back'-powering the USB when (V)Bat / VCC higher than 4.7V. When (V)Bat / VCC though lower and board plugged into USB with power, USB power will charge the battery. This becomes a roblem for the PCB tracks when the charging (or 'short'-cut) current goes beyond... (I d not know how the tracks are reated. From looking at the board, the connection from/to 0.1" (V)Bat / VCC pin is much fatter than the one from/to 0.05". You can prevent charging by putting a diode between the battery and the (V)Bat / VCC pin, but that's not much different then using the BAT_IN / Bat pin over the P FET, but with less options.
@alexanderbrevig concludes that the (V)Bat is the place to connect a bat from 3.5..16V... and see no issues when not connected to USB power/USB connector. When connected to USB Power, it can become an issue when the (V)Bat conneted battery starts to draw current (Battery delivers less than 4.7V).
With other words, BAT_IN is safe for sure, and V(BAT) is safe as long there is no USB POWER connected. Therefore, when you do not plug it into a powered USB porte (kond of an oxymoron by the definition of the USB port...) you can use either.
Putting the FET/B0 solder jumper in, makes it a different ball game.