I need some help understanding the pins of I2C 8-bit expander (PCF8574AT) that looks like this,
In the datasheet for this part, there is a pinout diagram for the 16 pin header but nothing is mentioned about the four pins on the top of the module which is SDA, SCL, VSS, and VDD but these already exist in the 16 pin header.
Datasheet for the expander, https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf
Please help me understand why there are two sets of these pins. Basically, I want to understand how the on-chip IC of the module is wired to the header on its side plus the header on top.
Edit: Added datasheet. Edit: Clarification.