I have the pin-out assignment AND a suggested use for the J2 header.
Raspberry Pi 4B
"J2" – 3-pin HEADER PIN-OUT
Pin 1 (square; "Global_EN): ---- 5V
Pin 2 (round; middle): ---------- Gnd
Pin 3 (round; "RUN"): ----------- 3.3V
The J2 Header connector CAN BE USED FOR A CASE-FAN CONNECTOR, after soldering wire pins to empty J2 header holes on on PCB...
NOTE: The center connector is direct-connected to Ground pins (Gnd) on the GPIO header, since Ground ("Gnd") is always common circuit-wide, whether a device is powered or not. However the outside J2 Header pins, 1 and 3, do NOT have ohmic continuity to GPIO power voltage pins - but the J2 header's 3.3V and 5V pins ARE active when the Pi is powered. This does not lessen their usefulness as fan-connector pins, obviously. Use of the J2 header to connect a case fan is preferable to using GPIO pins for a case fan, if you intend to use a 40-pin cable to connect the GPIO header outside the case, and still have a case fan running. I personally would like to know what the J2 header is intended for, and am continuing to research this; it might not have wide applicability, since no header wires are connected to the empty J2 PCB holes. In the mean time, I'm planning to add header wire pins to the J2 PCB holes and use it to power my case fan (pre-installed in a Vilmos case), which is intended to run on both 5V (fast) and 3.3V (low) speeds. The Vilmos case fan comes with female header sockets attached to the fan wires, for plugging into GPIO pins. GPIO is NOT IDEAL for connecting a case fan, since I have a 40-pin female header with attached cable that takes GPIO outside the case (for breadboarding, etc).
[NOTE: The PoE header (Power-over-Ethernet) does NOT provide 3.3V and/or 5V power pins; they connect to transformer center-taps in the Gigabit Ethernet circuit block.]
REFERENCE: See the "Rasberry Pi 4 Model B (Reduced)" Schematic Diagram, available as a 1-page PDF download (my download was dated June 18, 2019):
www.raspberrypi.org/documentation/hardw ... educed.pdf