What is the diference between S80 and d grade shield?They dont add p def and have same def rate.

The P.Def of your shield isn't added to the amount of P.Def in your status window because the shield's P.Def only applies when the text message: "Shield defense succeeded" appears when you are attacked.

say you have 500 P.Def, and u're using a Kite Shield (142 P.Def) when u're fighting and no msg appears, your P.Def is 500, but if the msg "Shield defense succeeded" your P.Def will be 642 for the next hit you'll receive.
If you're using a Dynasty Shield (321 P.Def) your P.Def will be 821 when this msg appears, if not, it'll still be 500.

The previous example does not include shield mastery passives or buffs, the effectivity of your shield can be enhanced with this passive, or the advanced block buff (think of it as a "Shield" buff for your shield).
The shield succes rate can be enhanced with the Bless Shield buff and the shield mastery passive.

every shield has its p def that is added when blocking attack. so you dont see this extra p def in stats.

Very good explanation. That's correct.
L2calc also sais that upon blocking a magical attack, the shield pdef counts too.

If I remember correctly, there are 2 messages:
1) Shield Defense succeeded - Guys here already explaned, how does it work!
"Your shield defense has succeeded."
2) Shield Defense completely succeeded - block 100% damage - very rare!
"Your excellent shield defense was a success!"

Perfect shield defence occurs with an increased chance when being attacked by a bow. I did some tests back in interlude with one of my buddies. Apparently, there were 11/100 perfect blocks while attacked by a dagger and 19/100 perfect blocks when attacked by a bow. I don't know how accurate those tests were because we only did one set, but i'm still inclined to say that you have more chances to block an arrow. Btw, those tests were done on an sk, with bshield lvl 6 + guard stance. 100% block rate for short.