Did you know that sequins have a front and a back? Well you do now. ;-)
Cupped sequins are easy to figure out. You have an upsidedown side and a cup side. You can stitch them either way, although the cup up is probably more common and more traditional (and also a tad more durable in my opinion).
But flat sequins are a bit more subtle. Generally, there is a shinier side and a duller side. Or perhaps one side is more vivid in color, with the other side duller in hue. It can be hard to see the color/glossiness difference, but it's usually most obvious on flat, vintage sequins. Here are a couple of photos:
Traditionally the brighter, glossier side is considered the front. But you can play with using both sides to create nuances of color in your work. My advice would be to be consistent about how you use both sides, and pick a stitch or application where it's really going to show and won't look like an accident.
For example, if you're filling an area using overlapping rows, do one row using the fronts and the next row using the backs.
You can check out my other tutorials and posts about sequins by clicking "sequins" in the Topics list.