The default I2C address is 0x70.

Power Pins

V+ - this is the power pin. Since the multiplexer chip uses 3-5 VDC, to power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V micro like Arduino, use 5V
This connection is shared with the STEMMA QT port on the opposite side of the board.
- - common ground for power and logic.
This connection is shared with the STEMMA QT port on the opposite side of the board.

Vlogic Switch

Vlogic - this switch selects between supplying the 8 STEMMA QT ports with either the incoming voltage on V+ (3-5 VDC) or level shifting to 3.3V 500mA (labeled 3V). For example, if you're using this with a 5V microcontroller like an Arduino 328-compatible, you can level shift all the QT ports to have 3V power and logic level.

I2C Logic Pins - Control

SCL - I2C clock pin, connect to your microcontroller I2C clock line. This pin is level shifted so you can use 3-5V logic, and there's a 10K pullup on this pin. This connection is shared with the STEMMA QT port on the opposite side of the board.
SDA - I2C data pin, connect to your microcontroller I2C data line. This pin is level shifted so you can use 3-5V logic, and there's a 10K pullup on this pin. This connection is shared with the STEMMA QT port on the opposite side of the board.
STEMMA QT - These connectors allow you to connect to dev boards with STEMMA QT connectors or to other things with various associated accessories. There's one port at the end that connects to your microcontroller. The other eight connectors in two rows of four are discussed below.

I2C Logic Pins - Multiplexed

STEMMA QT Ports 0 to 7 - There are eight JST SH 1mm connectors in two rows of four, all with the power, ground, and SDA/SCL pins connected. There are 8 sets of SDA and SCL pins, from SD0/SC0 to SD7/SC7. These are the multiplexed pins. Each one is a completely separate I2C bus set. You can have 8 I2C devices with identical addresses, as long as they are on one I2C bus each. The power input for the connectors is the output from the Vlogic switch, and is either V+ or 3.3V 500mA.

These ports do not have any pullups installed, so if you are using a chip or breakout without I2C pullups be sure to add them!

Address Pins

On the back of the board are three address jumpers, labeled A0, A1, and A2, between the board name and Adafruit logo on the board silk. These jumpers allow you to chain up to 8 of these boards on the same pair of I2C clock and data pins. To do so, you solder the jumpers "closed" by connecting the two pads.

The default I2C address is 0x70. The other address options can be calculated by “adding” the A0/A1/A2 to the base of 0x70.

A0 sets the lowest bit with a value of 1, A1 sets the next bit with a value of 2 and A2 sets the next bit with a value of 4. The final address is 0x70 + A2 + A1 + A0 which would be 0x77.