Electronic – Using piezo buzzer to play complex waveforms

They're called self drive types, and they're meant to be used as part of the oscillator:

The piezo effect works both ways: if you apply a voltage the piezo stretches, but also if it stretches it creates a voltage. This principle is used to create a feedback signal which drives the oscillator.
The advantage of the self drive is that it will automagically work at its resonance frequency, where it produces the loudest sound. In 2-wire circuits the oscillator's frequency is independent of the piezo's resonance frequency, and it's the designer who has to make that they're close.

For the piezo of your picture:

"G" = black
"M" = red
"F" = blue

(I guess M, F and G stand for Main, Feedback and Ground, resp. CMIIW)

The industrial PCB assembly process usually leaves residues — mostly soldering flux — on the circuit board. One step in the process is to wash the board (by dipping or spraying) with a solvent to remove those residues for long-term reliability and for the sake of appearance.

Some devices (such as sound or pressure transducers) have openings for their functioning, and their performance would be adversely affected if the solvent or the residues got washed into the opening and lodged there. Therefore, such devices often have a sticker that covers the opening(s) that should not be removed until after the washing.

Removing the stickers adds an extra step to the process, so for really high-volume manufacturing, it is often worthwhile to select parts that are declared "washable" to begin with.