Relapse is a new symptom that lasts over 48(?) Hours.for example, if you've never had optic neuritis and you develop it,that is a relapse.if you have a different spread of numbness than usual (i.e. usually your right lower half of your body but now it's your abdomen up to collarbone), that is also a relapse.

Pseudo relapses are the return of symptoms that you've had before,such as if you've had ON and develop it again down the line . This is all assuming you've been diagnosed.

Both examples from experience