The multi-trainer version has 14 teeth and is mounted on a freehub that can’t be used on a Tacx trainer. The Elite & Garmin version has 18 teeth and is not mounted on a freehub body. On the Tacx trainers, it is OK to use the 14 tooth cog, whereas the Elite trainers need the 18 tooth cog to achieve low Watts at a reasonable cadence. So you can order the multi-trainer version, and remove it from the freehub body using a Shimano Hyperglide lockring tool and a chain whip, then install it on your Tacx freehub. If you don’t have those tools or don’t want to buy them, you could take the Cog and the trainer to a bike shop and they can install it for you in about a minute.