What are the functions of the test pads on the Pi 3B/3B+?

Question

The various models of the Raspberry Pi comprise numerous test pads (testing pins). Many of these pads however are not mentioned or properly labelled in the official reduced schematics the Raspberry Pi Foundation provides.

What are the functions of these pads?

Answer 1

Testing points on a Raspberry Pi 3B and 3B+:

PP1   VCC +5V (from micro USB receptacle) 
PP2   VCC +5V (from micro USB receptacle)
PP3   GND
PP4   GND
PP5   GND
PP6   GND
PP7   5V after polyfuse
PP8   3V3
PP9   1V8
PP10  Power LED
PP11
PP12
PP13  Status ACT LED
PP20  H5V (5V at HDMI connector)
PP21  RUN (reset signal)
PP22  Ethernet Activity LED
PP23  Ethernet Link
PP24  Composite Video
PP25  L Audio
PP26  R Audio
PP27  USB VCC
PP28
PP35  VCC +5V
PP36  USB D-
PP39
PP40
PP41  USB D+
PP42  USB D-
PP43  USB D+
PP44  USB D-
PP45  USB D+
PP46  USB D-
PP47  USB D+
PP48  GND
PP49  GND
PP50  GND
PP51  GND
PP52  Ethernet TX+ ?
PP53  Ethernet 3v3 ?
PP54  Ethernet RX+ ?
PP55  Ethernet TX- ?
PP56  Ethernet RX- ?
PP57  Ethernet 3v3 ?
PP58
PP59
PP60 

Sources:

• https://sudomod.com/forum/viewtopic.php?f=23&t=1491
• https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/ (reduced schematics of Raspberry Pi 3 Model B+, Revision 1.0 and Raspberry Pi 3 Model B, Revision 1.2)

Further reading (concerning Pi 2B and B+):

• https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=89522
• http://ozzmaker.com/testing-points-raspberry-pi/