[DerekAsirvadem]

I had to read your posts twice, slowly, I now think I understand what you want. The image you've posted isn't what you want, right ? You want that image, plus changes:
• the rectangular ribbon rendered as a trapezium
• whether you treat the objects as a group or not, is not at issue
• the content of the white square and the image are not relevant
Right ?

In that case it is simple, and sorry but the terminology did get in the way. And no, you haven't found the operation, so whatever those manuals called it, they did not work for you, right ? And it is definitely not "masking".

Ok, there are several ways to do that. If your example is typical of your need, the simplest and fastest is to use a trapezium, and place the label "sale" in it; then rotate it 45º. Then place it wherever you like. To get that exact shape of the required trapezium, Edit/Shapes/MakePointsSelectable. It helps if you are using a grid and the image (or white rectangle or whatever) is first aligned to the grid.

You can also just take your existing rectangle; rotate it; Edit/Shapes/MakePointsSelectable; change the points to make it a trapezium.

You can do it with SubtractShapes but you need a different approach and a bit of experience with OG.

>> Is this achieved by grouping the shapes and then re-ordering them so that one shape clips/masks another?

No, but that is the correct approach for obese products that are passed off as drawing tools, or not using the right tool for the job. Kinda like using PhotoShop to layout a catalogue, a bit silly, you end up with a very fat file.