3. L4T

L4T stands for "Linux for Tegra."

In this site, L4T usually means the folder generated by NVIDIA SDKManager for a specific Jetson model with a specific JetPack version. L4T contains the tools for flash and downloadable sources to recompile kernel and device tree.

For example:

JetPack 4.4 + Jetson AGX Xavier Module is located:

$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_AGX_XAVIER/Linux_for_Tegra

JetPack 4.4 + Jetson Xavier NX Module is located:

$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_XAVIER_NX_DEVKIT/Linux_for_Tegra