Vegan

It's high protein, it just doesn't have a perfect amino acid profile. Basically if it's your only source of protein/aminos, you could become nutritionally deficient.

Specifically, it's sorta low-ish on Lysine and Tryptophan. It's not that there's none in seitan, it's just not quite in the quantities you'd want if you fully used it as your sole protein source.

As long as you eat some other food that's known to have those two amino acids (which many plants and other protein sources do), you're good :)