DCIN and USB-5V are the names used in the schematics. The actual behavior of these pogo pins is not obvious based on their names. DCIN is connected both to the VBUS line of the USB Type-C connector and to the ACIN/VBUS inputs on the PMIC. This means that, depending on a number of factors, DCIN may be at 0 V or 5 V. USB-5V is connected at the output of an LP6226 DC/DC boost converter (5 V), which in turn is fed by the PS output of the PMIC. The boost converter is enabled or disabled by a GPIO output from the A64 SoC, controlled by software (e.g. the Linux kernel). Depending on inputs and decision made by the PMIC, PS may be at the battery voltage (fed "directly" by the battery through a transistor controlled by the PMIC), or at the "USB" voltage (fed by the PMIC's ACIN/VBUS inputs). This means that depending on a number of factors, USB-5V may be at battery voltage (between 3.0 V and 4.3 V), or at 5 V.
Because the PinePhone may act as a USB host (providing 5 V at the USB Type-C connector's VBUS to a connected device) or as a USB device (drawing from a 5 V source on the USB Type-C connector's VBUS), DCIN is actually not strictly an input nor an output. Some community analysis of the PinePhone schematic (and some testing) indicates that you can connect a 5 V power supply to DCIN in order to power the phone at the PMIC's ACIN/VBUS inputs (and, as a side effect, charge the battery). This may not be safe to do in all conditions, e.g., when the phone is acting as a USB host to a connected USB device. It should also be safe to use DCIN as a power output from the PinePhone, e.g., when a USB Type-C charger is connected, you can draw current directly from the USB Type-C port's VBUS, which is provided by the charger. Please note that, when using DCIN as an output from the PinePhone, DCIN isn't "always on"; it may be 0 V. It is currently not documented on how much current can be safely drawn.