Answer: please see the following.

Mimaki Inks have an expiration date on them to avoid poor print quality. Printer displays the message, [Expiration] to let you know that installed ink passes the expiration date.
Replace the expired ink cartridge.

[How your printer operates in expired ink]
Your printer displays the message [Expiration] in the next month after the ink expiration; and even if you do not replace the expired ink, you still can continue to run it 2 months after the expiration, and when it passes three month from the expiration date, your printer ceases to operate.

[NOTE]
If you keep using expired inks, it may result in poor print quality such as degraded colors; we highly recommend for you to replace the expired ink any sooner.