Industrial PCs are computers meant to go in industrial environments. They vary quite a bit but often have to handle dirty (possibly explosive or corrosive) air, 24v power, wide temperature ranges, lots of electrical and radio noise, and being cabinet mounted.
If you've got a little Node-RED box reading serial data from your bar code reader, doing lookups in your SAP database, and then sending Modbus commands to your PLC to redirect a box down a different conveyor line, it's probably an industrial PC.
Streacom makes fanless and air-cooled cases computers that aim to look good. Their products target an audio/video and enthusiastic pc building market. I would say they would be on the same category as Silverstone.
Yeah, that sounds quite different than industrial PCs, where the engineer specifying it spends 10 minutes pouring over specs and 2 seconds looking at the picture.
If you've got a little Node-RED box reading serial data from your bar code reader, doing lookups in your SAP database, and then sending Modbus commands to your PLC to redirect a box down a different conveyor line, it's probably an industrial PC.