It is a zabuton designed for a gift in celebration of someone’s 99th birthday, which generally comes in white. If you take the part ‘一’ (meaning 1) from the kanji ‘百’ (meaning 100), it will be 白 (meaning haku). That’s why the celebration of the age of 99 is called hakuju.
The honoree is supposed to wear white garments such as a haori or a sleeveless vest and a bouffant cap, like the celebration of kanreki.