Although not stated specifically, historically this has been interpreted that engine debris cannot penetrate and pass radially through any of the engine cases, but debris can exit axially out the front or back of the engine.
Of course the point of all this is that blades exiting radially threaten the cabin, the fuel tanks and the hydraulic and electric control runs in the wings.
Bits of engine exiting forward or backward don't. They fall behind the aircraft in flight.
That's why the distinction is important.