I know what the PPI website says, but as others have said, the discipline aspect for the FE does not matter. They break it into disciplines to help those that are better at one field of study than the other. People that work in Structural might not do so well on the more specific afternoon Electrical questions.
Once you have your FE or EIT or whatever you want to call it, you'll get your required 4 or 5 years experience and then take the PE. That exam is the one where certain states require you to specify your discipline. In Delaware, you can be any type of engineer you want if you pass any discipline of the PE. In Alaska, you can only be a Civil Engineer if you take the Civil discipline.
And if you're still not sure, take whatever FE discipline you're most comfortable with and you think you can pass with. Some states won't count your experience until after you passed the FE. At least this way you'll start to gain the experience requirement.
If you still don't know in 4 years where you want to practice, find a state like Delaware that has no restrictions to take the PE in. That way you'll have the PE. But even with a PE from another state, you might not be able to get reciprocity into other states, as some states have different requirements. At that point though, you'll have the PE and you won't need to worry about all this FE/EIT stuff. No state is going to make you retake the FE after you've gotten a PE in another state.