Gcc: reload pass and "reload_completed" in backend.md files

I am working on a new backend for GCC and am getting help from existing machine description files for various purposes.

So far I don't understand the following:

• What does skipping reboot do?
• How does he interact with the combine? I see a lot of "reload_completed" errors in the MDC files of the MIPS file. Why are some define_insn_and_split templates with "reload_completed" and others without?

I've tried reading about what overloading goes through here and also tried some archive searches on gcc-help. However, what the reload is, what it achieves, and its interaction with other passes, such as the harvester pass, is not clear to me. Can anyone elaborate on this in more detail with examples?