For a smooth, real surface $\Sigma$, its bundle of symmetric, bi-linear forms $S^2T\Sigma$ reduced to a $PGL(2,\mathbb{R})$ structure. A similar reduction(with different structure group) can be done for other tensor bundles.
Where can I find a good reference on this type of reductions?
$PGL(2,\mathbb{R})$ acts by conjugation on the space of square matrices $M(2,\mathbb{R}).$$M(3,\mathbb{R}).$ Can one find any invariant polynomials associated to this action?