Gussets are either boards, squares or triangular shaped pieces of plywood used to strengthen joints. They can also be used as a bumper to help protect any hardware on the crate

In the design you can specify type and quantity of gussets and where you want them placed based on the individual panel. Pre-filled in are the three common type of gussets:

L = Corner gusset
I = Edge gusset
X = Square gusset

The name or item ID helps users define placement, so while you have the same shape used for your corner and your edge gussets, by using the letter designation you are telling your staff the expected placement. The X, I and L designations are based on the shape of the cleat boards the gusset covers. The center gusset covers the X shape of the cleats, the L is the shape of the corner and the I is for the edge but T is also commonly used. As a user you may rename the materials based as desired.

This link is to the Gallery of Styles with images of the gussets. Often you'll see corner gussets as triangular. Edge gussets usually take their shape based on the cleat dimensions they're covering but companies will also use triangles as well.

Remember that when you add a NEW item such as a gusset, check the gusset checkbox before you enter the thickness of the material.