Ok, so back with a schematic!
So, when might you use the top row of pins on the power configuration header to tie rails together?
On the 2x3 male pin power header, the second jumper setting on the top row for tying the 5V rail to Vin looks like this:
5V Vin 3v3
[o o] o
[o o] o
5V Vcc 3v3
You might want to do this if you were supplying a regulated 5V through the barrel jack connector, instead of the usual 6V+ supply.
In that case, tying the 5V rail to Vin would bring the 5V rail up to the full 5V level. Otherwise, it would be less than 5V, as there would be a voltage drop from 5V on the rail due to the LD1117v50 regulator.
Also note that in this case, you could dispense with installing the onboard 5V regulator entirely, if you wanted to. Note the 3v3 rail would still be supplied as normal by the LD1117v33 regulator, assuming that's installed. So you would still have the choice of running Vcc at 5V, as above, or at 3v3, as below:
5V Vin 3v3
[o o] o
o [o o]
5V Vcc 3v3
Analogously, you could tie the 3v3 rail to Vin directly if you were supplying regulated 3v3 via Vin. In that case, you could dispense with both onboard regulators, if so desired. (Of course, 5V wouldn't be an option for Vcc unless there was some other source feeding the 5V rail.)
5V Vin 3v3
o [o o]
o [o o]
5V Vcc 3v3
So there are a couple of the more specialized power configuration options available. Anyway, the design philosophy is that flexibility is a good thing, as long it doesn't lead to confusion or makes it too easy to shoot yourself in the foot. Hopefully this design provides a good balance.
SkinnyBob1284p.pdf (22.6 KB)