The concept of the “fandisc” is a curiously Japanese phenomenon that allows fans to engage with their favourite works in alternative ways, and for creators to celebrate the success of a work without making a full-blown sequel.
The closest equivalent we probably have here in the West is downloadable story DLC or expansion packs for popular video games, but those aren’t quite the same thing as a fandisc; while exceptions exist, they tend to be about “adding value” to an existing product, whereas your typical fandisc stands by itself as its own discrete title in the context of a larger series.
Such is the case with Nekopara vol. 0, an all-ages fandisc for the series that launched in August of 2015, about eight months after the surprise success of vol. 1.