Tegra 21x is the Tegra X1 series (I don’t know which variants exist, but typically the naming convention for a Tegra chip might be a tegra210 variant, tegra211 variant, tegra212 variant, so on, with each being a Tegra X1). Tegra X1 is an SoC distributed on the TX1 module. The module seats on the developer carrier board…I’m not positive, but I think when you mount the module on the developer carrier board is when you can call it a Jetson instead of a TX1 module using a Tegra X1 SoC. Third party carriers use the same module with the same SoC, but are no longer called a Jetson.
Tegra X1 and Jetson TX1 are the same processor, but differ in carrier board (the TRM is correct on both of those…board support package will differ).
If you go here, check “Jetson TX1” under product, then search for “TRM”, you’ll get the doc: