Technically, there are 2 card readers. The microSD card reader though requires ejection of the card tray to get at it. With that said, it is best to treat the microSD card reader as if it was an upgradeable storage device. The card must support U3 standards or Senitrus will warn you about performance issues with the lesser cards. The card tray is on the left side, and in front of the USB-C port(s). The SD card reader will be on the front right side to make it easier for most people to access the card slot. You would inject the card fully into the reader. When you wish to eject, you would want to unmount first, and press on the card further in to release the spring, and the card will pop out at about 50% so the card doesn’t fly out of the computer. For those that seen how floppy diskettes ejected from the reading drive, you would see how it will work here.
W and A lineups will support SDXC which will support capacities of up to 2TB?. Currently, this is the highest card I seen, and therefore will likely be an option for those that can afford the 2TB cards. The P lineup will support SDUC cards which can go up to 128TB of capacity. Currently, there are no SDUC cards I know of, and I will imagine the first ones will be 4TB. When these cards come out, you will not be able to use them unless you have the P lineup. This means 2/3 of the computers will not be able to support this higher capacity. SD cards are still used in digital cameras, and other devices. They can also be used to pass files that you wouldn’t want to send on the internet.