A slice is a portion of image of 16 lines x ( n x16) pels.
Each slice is coded indipendently from the other slices of the picture.
Therefore the slice layer allows error confinement because, when errors in the bitstream are detected, the decoder can try to continue the decoding process looking for the next slice header.
Video decoding process at the slice layer
Decode slice_vertical_position.
Decode quantiser_scale_code.
Decode all the macroblocks that compose the slice.