I'll throw in my two cents. In the state of Florida, the line is nearly impossible to define. I contacted the state board and got back an email response from a lawyer. It was written very lawyer-y - lots of wiggle room. These were the first two sentences of his response: "I have no definitive answer for you. There is no statutory bright line between architects & engineers other than PEs cannot call themselves architects when they provide design services." I took his repsonse to mean that if I can show that I had to engineer something in the building and I am competent to do other phases, then I could provide the entire design. I'm not going to take on more than I can handle, but if a client needs plans for a PEM or a light commercial building, I've got no problem with taking the job. If I need to sub something out to an SE or an EE, I do it.
Other states are probably different.