FR-I
FR-I radio source (3C 31): edge-dimmed, >50% flux within half full extent. Jet already sub-relativistic on intermediate scales. Jets dissipates as it moves out.
FR-II
FR-II radio source (3C 175): edge-brightened, ?50% outside half full extent. Jet remains relativistic to large radii, moving freely until it encounters IGM.
FR-I sources are lower power than FR-II, though the division increases for more luminous host galaxies. It seems the extent of the ISM affects whether a jet can get out of the galaxy .freely.