Where do automorphic Maass forms come from?

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I know the Langlands program for GL2/SL2 gives some hints as to the origins of automorphic representations, and that some cases of the correspondence have been completely classified or reasonably worked out, e.g. the discrete series representations and principal series of eigenvalue 1/4. My question is whether there is any conjecture regarding the origins of principal series of eigenvalues greater than 1/4?