It is a zabuton designed for a gift in celebration of 88-year-old birthday or 90-year-old birthday.
“Beiju” means celebrating the age of rice.
It comes from being able to read eighty-eight if it breaks the character of “rice”. Yellow is regarded as the color of a long life celebration.
“Sotsuju” means a celebration of 90 years old.
This is derived from the fact that the abbreviation of a kanji of “Sotsu” can be read as 99.