On the relation between standard and mu-symmetries for PDEs

We give a geometrical interpretation of the notion of mu-prolongations of vector fields and of the related concept of mu-symmetry for partial differential equations (extending to PDEs the notion of lambda-symmetry for ODEs). We give in particular a result concerning the relationship between mu-symmetries and standard exact symmetries. The notion is also extended to the case of conditional and partial symmetries, and we analyze the relation between local mu-symmetries and nonlocal standard symmetries.