A paper by Kaczorowski & Perelli [arXiv:1207.2312](http://arxiv.org/abs/1207.2312) dealing with the elements of the Selber class with degree two suggests that $S\_2$ coincides with the automorphic l-functions over $GL\_2( \mathbb{Q} )$.


**EDIT.** We know from [[1]](http://www.ams.org/mathscinet-getitem?mr=1253620)[[2]](http://link.springer.com/article/10.1007%2FBF02392574)[[3]](http://arxiv.org/abs/0804.0715):


$\mathcal S\_0=\{1\}$


$\mathcal S\_1 =\{\zeta(s),L(s,\chi)\}$ $\chi$ primitive plus shifts.


$\mathcal S\_2 =\{\zeta^2(s),L(s,\chi\_1)L(s,\chi\_2),L\_f(s),\zeta\_K(s)\}$ primitives, shifts (list only conjecturally complete)


Can be those classification results proved for automorphic representation on $GL\_n( \mathbb{Q} )$? Improved perhaps for $\mathcal S\_2$? References? Conjectures for arbitrary n?



Automorphic L-functions over $GL_n( \mathbb{Q} )$
I think your question is too broad, at least it is not clear to me what you are really asking. At any rate, if the Ramanujan-Selberg conjecture is true, then the $L$-function of an automorphic representation of $GL\_n$ over $\mathbb{Q}$ (with unitary central character) belongs to the Selberg class. Moreover, it is believed that these are all the elements of the Selberg class, hence in particular every element of the Selberg class should satisy the Generalized Riemann Hypothesis (I think this was Selberg's original motivation). The $L$-functions coming from cuspidal representations should agree with the set of primitive elements in the Selberg class. The extended Selberg class is definitely larger, because it is closed under complex linear combinations. You can find further information in the surveys of Perrelli, e.g. in [this one](http://link.springer.com/article/10.1007/s00032-010-0119-2).


Note that the Ramanujan-Selberg conjecture is only known in very special cases (e.g. for certain automorphic representations of cohomological type). On the other hand, the relevant axiom of the Selberg class can definitely be weakened without really enlarging the class (as far as we know), and then the principal automorphic $L$-functions mentioned above provably belong to the modified class.


