Electronic – what does this schematic symbol refer to?

The Electrical Rule Check executes, depending on the pin direction (specified by the author of the schematic symbol), various checks. It expects for the direction of type Pwr a Sup pin set for this net. Sup is a pin type for power supply outputs for ground and supply symbols.

From the Eagle manual:

"[...] For every Pwr-pin there must be at least one pin with the same name but the direction Sup(a supply pin). There must be one on every sheet. These Sup pins are fetched into the schematic in the form of power supply symbols, and are defined as Devices in a library (see supply*.lbr). These Devices do not have a Package, since they do not represent components. [...]"

Further...

"If there is no supply pin in the supply libraries that fits to your voltage in the schematic, you have to define a new supply pin! Renaming an already existing supply pin is the wrong way and can lead to unexpected results!"

When using Eagle's device editor, you can use 'append' to connect multiple pins to one signal. By default eagle expects all pins to be connected together, but by clicking the little icon to the left of the set of pins, you can toggle between 'all pins' and 'any pin' required for the connection.