"Sub weapons" refers to smaller, secondary weapons that are used in addition to a main weapon. They are typically used as backup or for specific purposes in situations where the main weapon may not be suitable. Full definition
You'll unlock various sub weapons to help take them down too, including the likes of a bow and arrow or bombs.... (useapotion.com)
So selecting your weapon is like striking a balance between the attack style, how you use the weapon as well as finding sub weapons you like and find useful. (darkainarts.com)
Your special weapon is determined by your main weapon, and includes everything from a protective bubble, using a barrage of sub weapons without having to use your ink supply, one reveals enemy positions, another launches a tornado of ink at any location you desire by using the GamePad screen, and so on. (consoleob.com)