The summer uniform for the students of Nekoge Prefectural High School. For the female students, the uniform consists of the normal pink tie, but it has a blue vest instead of the green jacket or sweater. For the male students, they wear short-sleved button downs.
Io mentions that it looks good on Tsumiki, and Mayoi uses the more exposed button-down as a chance to flash Io with Tsumiki's chest/bra, which she calls "Summer Uniform Fanservice".