This emoji means “full” in Japanese, usually used to denote “full capacity” such as a gas/petrol tank on a car or pre-paid phone card.
CJK Ideographs are characters used in Chinese, Japanese and Korean languages. Their writing systems all completely or partly use Chinese characters.
Japanese “no Vacancy” Button was approved as part of Unicode 6.0 in 2010 under the name “Squared CJK Unified Ideograph-6E80” and added to Emoji 1.0 in 2015.
🈵 Squared CJK Unified Ideograph-6E80