400 GB is no longer the biggest microSD card available. As of some time in February 2018, there will be a (very likely expensive!) 512 GB microSD card on the market! : r/gpdwin
Markings: speed, size and class | Understanding microSD and SD cards: speeds, markings and more | Adafruit Learning System
The Best Micro SD Cards For The Nintendo Switch - GameSpot
Everything You Need to Know about SD Cards | B&H eXplora