This breakout has a 2.9" tri-color (red, black, and white) display. It has 296x128 black and red ink pixels and a white-ish background. Using our CircuitPython or Arduino libraries, you can create a 'frame buffer' with what pixels you want to have activated and then write that out to the display. Most simple breakouts leave it at that. But if you do the math, 296 x 128 pixels x 2 colors = 9.5 KBytes. Which won't fit into many microcontroller memories. Heck, even if you do have 32KB of RAM, why waste 10KB?
So we did you a favor and tossed a small SRAM chip on the back. This chip shares the SPI port the eInk display uses, so you only need one extra pin. And, no more frame-buffering! You can use the SRAM to set up whatever you want to display, then shuffle data from SRAM to eInk when you're ready. The library we wrote does all the work for you, you can just interface with it as if it were an Adafruit_GFX compatible display.
For ultra-low power usages, the onboard 3.3V regulator has the Enable pin brought out so you can shut down the power to the SRAM, MicroSD and display.
We even tossed on a MicroSD socket so you can store images, text files, whatever you like to display. Everything is 3 or 5V logic safe so you can use it with any and all microcontrollers.