I'm no "expert" but I will share my .02 worth... Target ammo is generally FMJ (full metal jacket) rounds with a minimum load of powder. Upon impact, a FMJ bullet tends to keep it's shape and speed, giving more opportunity for the round to pass through the target and perhaps hit an unintended target, and also by not expanding the round delivers less energy to the target... less "knock down" power. Self defense ammo, generally is JHP, jacketed hollow point, and is often +p or +p+ loads, meaning more powder charge per round. This delivers mor energy to the target, and the JHP round flattens or expands upon impact delivering more "knock down" power and less chance of passing through and hitting someone or thing that is not you target.
You can use any ammo for self defense (I wouldn't want to be on the receiving end of any ammo) but there is real and valid reason to use the better defense ammo for that purpose. Buy one box and keep a magazine full for that specific purpose. Buy what's on sale for practice... maybe some hot stuff sometimes just for fun!