Ext 1 is crucial for Ext 2 if your aiming for the high bands because it teaches you a special type of thought process technique that is not available in 2u but is available in 4u and this thought process is really what takes you to the next level. But if your willing to stay shallow in 4u than...
It said the understanding of trace elements not the understanding of how aas works, like advantages are like human exposure to lead, through aas we are able to diagnose the symptoms and provide medicine, see that's how focusing on the understanding