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).