It is a zabuton designed for a gift in celebration of someone?fs 99th birthday, which generally comes in white. If you take the part ?e??f(meaning 1) from the kanji ?e?S?f(meaning 100), it will be the??(meaning haku). That?fs 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.