Hitting with different units in succession quickly enough starts a "Chain." The more attacks you connect together, the higher the "Chain Length" will be. Chaining two hits together is a chain length of 1, because the second hit is what causes the chain to start. If the same unit hits twice in a row, THAT chain is finished.

Hits that increase the chain length (aka the second hit on) have their damage increased by a modifier dependent on both the chain length and the type of chain it is. The modifier formula is...

Modifier = 1   (Chain Length * Chain Type Mod) | Max: 3x

Chain Types are

Element Chains only need a chain length of 7 to cap out at 3x damage (at 6 it's 2.8x). Normal Chains need a whopping length of 20 to cap, and by then most of your hits and damage will have gone underboosted. Spark Chains are the most rewarding but the only way to hit them is with frame perfect timing. As such testing multiple spark chains in succession has not been done yet. It is also unknown if you can change the type in the middle of a chain and retain the boost from the length, further testing required.

Frame Data on Abilities/Magic isn't on the wiki yet (next Tuesday maybe?) but consulting this in the future will help you see what chains well together or not. Or you can just wing it with high hit counts like Tidus and Orlandu.

tl;dr Element Chain Is B R O K E N

thanks /u/Heer0 for the video to find the cap https://www.youtube.com/watch?v=BPRVEmPxZUI