Look up Ed Ravenscroft in old MR's. He wrote several articles on tab on car back in the 60's and 70's. There are lots of ways to do them.. They can be a thumbtack which fits in a hole on the top of the car (removed with a magnet). Or there can be a discrete pin placed in the top of the car and a coded washer placed over the pin. Or they can be little I-beams with a move on each side and the car type on the edge. Or you can put a magnet in the car on the side of the car and stick a color coded square piece of steel or tin sheet on the side. Or you can glue a thin piece of sheet steel or tin to the side of the car (like a tack board) and then attach a coded magnet to the side of the car. In more modern times there are lower tack marker "dots" of many colors that can be attached to the top of the car.
Lots of options.
These methods are actually quite prototypical since it was common in the steam era days to "card" an inbound cut with the blocking codes on the tackboards. A clerk would get a switch list of the car and walk the track tacking the appropriate coded card to the tack board with a hammer stapler. These methods were used into the 1980's. For example the GH&H (Galveston, Houston & Henderson) was 50-50 owned by the MP and MKT. When a MP train arrived Galvez yard, a clerk would staple blue cards to the cars, when a MKT train arrived, they stapled tan cards. Then the cars were delivered to the Galveston Wharves RR. When they came back, they were switched MP or MKT by the color of the cards.
The advantage of the tag on car systems are they are simple to build and are not car number specific, which makes them great for modular set ups and clubs where the cars (and maybe the industries) will vary from session to session. The down side is that the tags can be considered "unsightly" by some and the idea of physically modifying the cars (hole for a thumbtack or pin for a washer) is a non-starter for others.