Some characters in the game have alternate versions, often time being much rarer and appearing in a different position (for example Sione being in the Gold position and Sione SP being in the White position). These characters can't be used together in the same team.
At the moment, there are two types of alternate version characters: SP characters and MZ characters
SP characters are rare, limited-time versions of original characters. SP characters are obtainable through limited-time infuses as well as Origin Infuse and Contract Crafting (for 50 Origin Stones), and are only available at SSR tier.
SP characters are "what-if" versions of original characters, often depicting original characters in different instances of time and scenario.
MZ characters, with MZ being short for mizugi (水着, swimwear in Japanese), are limited-time summer versions of original characters. They feature the original characters, but instead in a swimwear attire and have their character design based on summer activities.
MZ characters are only obtainable through limited-time Summer Infuse from 17 July to 20 August 2019 (with Naya MZ being only obtainable through the limited-time Summer Fantasy event), and are only available at SSR tier.