Your soul is basically eternal, it learns, it lives in the characters you play, and its a "character slot.”
The spark you use is your "subscription" to play that character, it is what breathes life into that character making it able to be played.
You have "Spirit" when you spark into a character that is approximately equal to 10-12 months.
As you "die" and are caused to spirit walk, this makes you lose some of that spirit and is called "Spirit Loss".
This results in less time for you to play that character, losing time off your Spark.
Once you reach zero spirit, that causes you to perma die, causing your soul to leave that characters body and return to the list to be "reborn" or "reincarnated"