Confusing language on arduino docs about powering via 3V3/5V pins

Question

I'm reading about powering alternatives for the arduino board, and I have run across two confusing sentences in Section 5, powering via 3V3/5V pin. First, it says:

3V3 and 5V pins can also be used as power inputs if no regulated power supply is connected through the other power inputs (USB port, barrel jack connector or VIN pin).

But then right afterwards, it says:

Although 3V3 and 5V pins can be used as power inputs, it is not recommended if no power supply is connected through the USB port, the barrel jack connector, or the VIN pin.

(emphasis mine)

Which is correct? When powering via the 3V3 or 5V pin, should I have a different power supply connected elsewhere, or not?

• jsotola: they are both correct ... one says you can do this ... the second one says but it is not recommended that you do this ... similar to you can run across the street ... but it is not recommended that you run across the street .... they are basically saying don't use the arduino as a power supply if you can avoid it
• 6v6gt: The issue appears to be backfeeding the voltage regulator which is connected to the 3.3v or 5v pins. Some voltage regulators don't like it (see electronics.stackexchange.com/questions/110314/…).

Answer 1

The second sentence ‘Although...’ is nonsensical. If the other ways to get power are not used, you can’t use the last possibility?

I have also seen it claimed in comments that the regulator is useless if you put 5V to Vin (when 7-12 is recommended.) An UNO LDO regulator spec sheet lookup has a minimum applied voltage closer to 3V - so it will regulate to a bit less than 5V when 5V is applied. I found this out in practice when I neglected to change supply from Vin to 5V pins. My LEDs lighted but not as brightly as expected. The LDO regulator did what it was told. The processor worked.