Nvidia's TU104 silicon is what powers the RTX 2070 Super, RTX 2080, and RTX 2080 Super. Being that a fully equipped TU104 die contains 3,072 CUDA cores, and the RTX 2060 runs 1,920 CUDA cores, we can infer that these TU104 dies are likely those that didn't make the cut for the higher-end cards.
Some yields are better than others, that's just the nature of lithography. Thus, Nvidia has to cherry pick the best chips for high-performing SKUs. However, chips that don't make the cut for more powerful models are free to be recycled for those that don't require a fully capable die. This allows Nvidia to profit from otherwise unused silicon, and it gives AIB partners more flexibility in pricing.
There shouldn't be any meaningful difference in gaming performance between a model endowed with the TU104 die versus one with the TU106 die. However, there are reports of improved workstation performance, such as Blender, if that's of interest. TechPowerUp also reports that the TU104 chip does drink a bit more power, too.
Being that there will be a mix of cards with both TU104 and TU106 dies, there will be no real way for consumers to discern which one they're getting prior to purchase.