Can I control exactly which SDS a QR poster shows using a custom field?

Yes, for advanced setups — a QR code poster's main job is giving access to everything at a location, but if you've set up an enumerated custom field (e.g. a "zone" or "line" value on your products), you can use it to further restrict which SDSs a specific QR code surfaces, beyond just its assigned location. This is a niche setup most teams won't need, but it's there for sites with tighter segmentation requirements.