Someone please explain this merc ias table for me

As far as I understand, action frames listed there are frames in the whole attack sequence where hit tests are done. Since act2 merc attacks twice per attempt using Jab, there're 2 action frames per attack frame sequence.
The simplest way to explain it is to give an example. Let's say that you have a sword, and you want to repeatedly swing it at me. First, you'd swing the sword at me. Second, you'd pull the sword back. Third, you'd swing the sword at me again. Fourth, you'd pull it back again.

Frames per attack is the total frames for the entire cycle- you swinging your sword and then pulling it back again- while Action Frame is the exact frame at which you hit me. Let's say it takes you 8 frames to swing, and 8 frames to pull the sword back. That means that you have 16 frames per attack, but you hit me on the 8th frame. That's 16 fpa, but the action frame is frame 8.

As for which table to use... it depends on your merc's weapon speed. Look up what speed weapon your merc has on Arreat Summit (it'll be the number listed in [brackets]), and then use the corresponding table.
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