What types of attacks can be shielded and what types can not shielded, and those that can't be (if they exist) can they be deflected/parried?
There are 3 fundamental types of attacks: M/R attacks (which deal K/E damage), F/T K/E attacks, and F/T I/E attacks. M/R attacks can be dodged or shielded (so Shield, Abs, and Def all apply). F/T K/E attacks can only be resisted or shielded (resist chance only comes from talents or set bonuses so the only stats you can get that improves this are Shield and Abs). F/T I/E attacks can only be resisted (so none of the tank stats are particularly useful against them).
In general, a vast majority of attacks are M/R (iirc, dipstik said it was roughly 80% weighted for damage dealt) with a smaller portion being F/T K/E (something like just short of 20%), and only the smallest margin of F/T I/E attacks (~1%). Even though Defense is less generally useful, because it has a smoother DR curve, you'll still stack a good deal of it.
I can't think of any fights with the exception of Kephess the Undying that are primarily F/T attacks of any kind. That's the only fight I can think of where the ratio of incoming damage is skewed enough that it might be worth it to swap out the defense relic for the abs relic, though, in all honesty, it's not a big enough concern to bother with unless you're *really* focused on making sure your relics are appropriate for each fight. Most tanks are just content to get their gear designed around the average incoming damage ratio: you won't be absolutely optimal for any one fight, but you won't have to change your gear for every single fight.