Yes, that's only for writing. For example, to collect all lines matching a pattern into a register with :g/pattern/y A you need to use an uppercase register

Yes, that's only for writing. For example, to collect all lines matching a pattern into a register with :g/pattern/y A you need to use an uppercase register ("A" in this case) because it executes `y` once per match. With `y a` instead of `y A`, you would end up having only the last match in the register.