There are multiple sources of power on the Mayfly boards. There’s 3.3v that’s always on, then there’s a separate 3.3v regulator that can be switched on and off by setting pin D22 high or low in your code. There’s also a separate 5v boost regulator (and on Mayfly v1.x) a 12v boost regulator, both of which are also controlled by D22 in your sketch. There’s actually no physical access to pin D22 to the users, it’s simply the trace from the ATmega1284P chip to the regulators that gets set high or low by your sketch. All 3 of the switched regulators are active-high, meaning setting D22 high will activate the regulators, and setting D22 low will deactivate the regulators.
You can access the switched 3.3v, 5v, and 12v power output lines in two different methods. One is through the 2×10 headers on either side of the Mayfly. Both headers have the same power and ground outputs: GND, 3.3v (constant). and the three switched power of Sw3, Sw5, and Sw12. These headers are used if you’re using a shield or mini-shield or accessory board that needs access to either the analog (left hand side) or digital (right hand side) pins.
Most people connect external devices to the Mayfly via the handy polarized 4-pin Grove jacks. The Grove jacks contain 2 data lines, one ground line, and 1 power line. The voltage of the Grove jack power line is determined by the position of the tiny jumper headers next to the individual Grove jack. Follow the label next to the jumpers to select the proper voltage you want for that Grove jack. The default setting as shipped out initialy is 3.3v, so move a jumper to the 5v or 12v setting to make the V pin of that adjacent Grove jack either 5v or 12v.
There have been multiple revisions to the Mayfly board over the past 7 years, resulting in different voltage output options and different jumper pin arrangements and orientations and labels. Details of these settings and photos showing non-default examples can be found on the Mayfly Jumper Settings page.