This is the heart of the manual. It details every simulation object available in the software. If you are trying to understand the difference between a "Mode Source" and a "Total-Field Scattered-Field (TFSF)" source, this is where you look. It explains:
However, one question pops up constantly on forums and Slack channels: "Where can I download the official Lumerical FDTD Manual PDF?" Lumerical Fdtd Manual Pdf
Ansys provides the Lumerical Help Center online. You can browse the FDTD section freely. While it is web-based, you can "Print to PDF" any specific chapter you need. This is the heart of the manual
The most common way to add custom functionality is through the Lumerical Scripting Language Custom Analysis Lumerical Fdtd Manual Pdf