This refers to the specific version of the mod. In development cycles, "d" usually denotes a minor revision or "hotfix" following version 0.16 to address specific bugs or compatibility issues.
The filename is a compressed archive typically associated with the Xenon Control Layer (XCL) , a community-developed modification or "mod" for the PC version of Xenoblade Chronicles X . Technical Breakdown XCL_0.16d.part01.rar
Adjusting draw distances and shadow quality. This refers to the specific version of the mod
This indicates that the mod is quite large and has been split into multiple pieces (multi-volume archive) to make downloading easier. You would need all parts (part02, part03, etc.) in the same folder to extract the full mod successfully. Context of Use XCL_0.16d.part01.rar