It is a zabuton designed for a gift in celebration of someone’s 90th birthday, which generally comes in purple like a koki-zabuton, kiju-zabuton or sanju-zabuton.
In some cases, a sotsuju-zabuton comes in gold or yellow.
The kanji ‘卆’, which is the simplified version of the kanji ‘卒’ looks like the kanjis ‘九十’ meaning 90. This is because we call someone’s 90th birthday sotsuju.
The thick type of zabuton is popular as a gift.