CAPSO! is a mechanic used for unlocking exclusive in-game content, including galleries, hit sounds (TapFX), character skins, custom songs, and bonus XP in Cytus II. It is the primary way to unlock Glitch difficulties of normal songs. It is earned by completing songs, which provides letters to the word CAPSO. When the full word is spelled out, 1 CAPSO! is obtained. A maximum of 99 CAPSO! can be held at once. It does not appear that there is any correlation between song performance and CAPSO! letters awarded.
CAPSO! is accessible by tapping the Black Market on the character selection screen. From there, tap the "CAPSO!" card on the left. While CAPSO! can be earned by completing songs, they cannot be opened without purchasing access to the CAPSO! system, which is US$9.99. CAPSO! are also not saved in cloud backups of game progress, though unlocked content is.
CAPSO! can be spent individually or in groups of 10. There is no benefit to saving them up (except EXP distribution), and there is a 99 cap on them, so it should be used immediately when purchased.
CAPSO! can reward players with new songs for Ivy (all of them have their Easy and Hard difficulty unlocked together, while Chaos is a separate pull), Glitch difficulties for certain songs (including paid character songs, which cannot be accessed unless the character is purchased), gallery images for (most of) Paff and NEKO#ΦωΦ's song artworks, tap FX (new sound effects for tapping notes), and character skins (new appearances for characters on the selection and results screens). All of these things are exclusive to CAPSO! (except character skins, with Vanessa gaining some as the story progresses).
CAPSO! can also reward players with EXP to help them level up characters faster (but will not provide EXP to certain characters, such as Vanessa and Bo Bo). There is also a chance for the player to receive nothing at all from a pull, where they receive a consolation prize of a few letters towards another CAPSO!.