About Fastboot & Recovery ROMs
The ROM packages you can flash on your device come in two formats: Fastboot ROMs and Recovery ROMs.
Fastboot ROMs come in .tgz files, they’re around 2GB in size, contain images for nearly all partitions and have flashing scripts for major desktop operating systems. Flashing a Fastboot ROM is very thorough, clean and flashes nearly all partitions on a device.
Recovery ROMs come in .zip files, they’re lighter and contain images only for the most important partitions. Flashing a Recovery ROM isn’t as clean as flashing a Fastboot ROM as it leaves many partitions untouched.
Official MIUI ROMs are available in the forms of Fastboot ROMs and Recovery ROMs. Recovery ROMs can be flashed using MIUI itself or XiaomiADB, while Fastboot ROMs can be flashed using MiFlash or the Xiaomi ADB/Fastboot Tools and they also need an unlocked bootloader. More info here.
Custom ROMs are available only in the form of Recovery ROMs and can only be flashed in TWRP. More info here.