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.