From what I understand you're supposed to get the result (in your example 6) by combining the cells in that box with the given operator (in this case times).

So a "6x" box with two cells results in (1 and 6) or (2 and 3), since those are the only products that result in 6. Likewise, "7+" with three squares can only be (1, 2 and 4).

7+ could also be 1,3,3 or 3,2,2 if the box is L-shaped so that the equal numbers don't go in the same row or column.

It is also important to note as it states in the article that order does not matter in these operations. Clearly it does not matter in addition or multiplication but say two boxes are outlined with "3-" the contents of the two boxes could be 4,1 or 1,4 in either order.