Section?
For a .hex file, I think it assumes that any discontinuity in the data starts a new "section." (I can delete a line from the middle of the contiguous code space, and suddenly I have three sections instead of two.)
For .elf and .o files, sections are a "big deal", and too complicated to explain here.
The optiboot implementation uses a separate section for the version number, which is what permits it to be place "at the end of Flash" while the code itself is placed "at the beginning of the bootloader section." (note that "bootloader section" is a hardware thing that is not particularly related to the .elf file "code sections.")