There were three variants to the AP-175:
AP-175P - Powered via 802.3at power over ethernet (PoE+)
AP-175AC - powered via external AC power source
AP-175DC - powered via external DC power source
The AC and DC powered models provide an 802.3af PoE power source (PSE) on the Ethernet interface. In this case, the port is labeled Power Out
The PoE powered model accepts 802.3at PoE+ power to source power for the unit. In this case, the port is labeled Power In.
It sounds like the 175 you have is either the AC or DC powered variant.