The magnetic screen lowers the outer magnetic stray field of the transformer. The screen is implemented by an encasement with highly permeable material. The material, e.g. mu-metal, can be built as a housing around the transformer or installed as a metal sheet. Magnetic screening usually doesn’t have a terminal.
Both ways of screening raise the dimensions of the transformer or lower the power at the same size.
Further terms to this topic:
EMC-transformer, mu-metal, mu metal pot, voltage disturbances, EMC-screen, special screen, capacitive screening, grid-to-plate capacity, permeability.