Intersection and overlap of right and near right angle edges

[AdamG]

Say I am making a box for a scupper or a tankwell for an SOT.. Can I tell if the panels or pieces meet "at the corners" or which part should end overlap the other? Are they measured to meet at the corners, more or less?

[jem]

Which model? All my SOT scupper have the same inner length and width.

[AdamG]

I'm building a Sabalo SOT. I haven't cut the scupper pieces yet, and I suppose it will be all clear when I actually go to put them together, but when you join pieces at the edge, along the edge, you can either put them corner edge to corner edge, or edge inside of end, and that makes a width of plywood difference in the dimensions.