I bought ASUS MeMO Pad 8 (AST21) from au, a vendor on KDDI CORPORATION, which seems to different from other ASUS MeMO Pad 8.
Then I terminated the device before I unlocked its SIM; so still locked.
At first its Android version used to be 4.4.2 (KitKat); I found and tried this article reviewing upgrating to Lollipop using unofficial zip file. (official upgrading things seemed not to be available any more; or it might not be available in the first place.) It changed my tablet's model number to K015.
Then I thought if its OS could be replaced, or if it could be upgraded more.
How could I enable flashing to booting section, or whatever appropriate?
fastboot flashing unlock or other flashing commands showed me FAILED (remote: 'unknown OEM command').
Both adb devices (works only when booted normally) and fastboot devices (works only on bootloader) recognize my tablet.
I am GNU/Linux user now; not Windows user.