3 products with a confirmed BMW application
Everything here is built for a BMW, so you aren't reading past parts for somebody else's car. Tell us the model and the year and you'll see only what fits yours. It comes down to the bolt pattern, hub bore and thread pitch, which come from your car's fitment profile, which is what the fit check is looking at.
A spacer moves the wheel outward by its thickness, which is the same as taking that much off the offset. That's the whole product, and there isn't more to it. Want a high offset wheel sitting flush with the fender? That's a spacer. Need to get around a big brake caliper? Same answer. It's also how you run one wheel size at both ends when the track widths differ.