What is the main contribution of Schneider and Shiffrin's theory to the field of attention?

The use of automatic processing strategies allows attention to be distributed or divided among various sources of information, with reasonable success in performing a task.
The theory establishes a fundamental distinction between controlled and automatic processes, where the former require conscious attention and consume attentional resources, while the latter do not present problems for attentional capacity and are difficult to modify once they have been established.