The Greatest Jigsaw

The image is a red herring because it is only used for assembling the puzzle. Once you've assembled the puzzle, it is no longer used.

How many unique symbols are there?

You need to fill in a number or symbol on each square of the 11 x 11 grid.

The number grid is a 11x11 Latin Square.

The Latin square by itself doesn't have anything close to a unique solution, so you need to get more constraints from somewhere else. It may be useful to think about why this puzzle was presented as a jigsaw puzzle.

It's useful to look at the shapes of the jigsaw pieces (in particular, look at the ones with 9s and 0s on them).

If jigsaw piece X pokes into jigsaw piece Y, and they both have digits, then the digit on X must be greater than the digit on Y. You can use this to solve the logic puzzle uniquely without any additional constraints on the sigmas.

The sigma suggests that you want to sum something up. Summing up all the adjacent numbers doesn't seem to give anything (not to mention that one of the sums is greater than 26), but that ignores the directionality of the pieces next to the sigmas.

Each sigma can be replaced by at least one number that satisfies the greater-than constraints from adjacent pieces. Sum up all possible replacement numbers and turn them into letters using A=1, B=2, etc.