As @oldturkey03 says, Q1403 is a power MOSFET (P-Channel enhancement mode MOSFET). This kind of FET is used for high-side switching, allowing PP_VCC_MAIN to charge the battery or not.
So in essence:
- PP5V0_USB gets converted to ~4.25V by the internal Buck converter in Tigris to create PP_VCC_MAIN.
- CHG_ACT_DIO will go low (0V) when it detects something in the charge port.
- VCC_MAIN & BATT_VCC are connected together via Q1403 and battery charging occurs.
- When the battery is fully charged, CHG_ACT_DIO goes high and cuts the connection between PP_VCC_MAIN & PP_BATT_VCC.
What's not entirely clear is what the battery lines in Tigris (A1-D1) do because we don't know what the internal logic is. Are they switched internally to output at VDD_MAIN (A2-D2) when no charging occurs or does BATT_VCC go though the body diode of the MOSFET to supply VCC_MAIN? Given that there is very little Forward Voltage drop measured across the body diode, I believe it is the former and not the latter. Only the Apple engineers know for sure.
I've edited this answer so that it is a little clearer and more importantly, there is a big unknown because no one knows what the internal logic of TIGRIS is. This answer is my "reading" of the schematic. Only the Apple engineers know for certain what goes on inside TIGRIS.