neovim/runtime/autoload

The autoload directory is for standard Vim autoload scripts.

These are functions used by plugins and for general use. They will be loaded
automatically when the function is invoked. See ":help autoload".

gzip.vim for editing compressed files
netrw*.vim browsing (remote) directories and editing remote files
tar.vim browsing tar files
zip.vim browsing zip files
paste.vim common code for mswin.vim and menu.vim
spellfile.vim downloading of a missing spell file

Omni completion files:
ccomplete.vim C
csscomplete.vim HTML / CSS
htmlcomplete.vim HTML
javascriptcomplete.vim Javascript
phpcomplete.vim PHP
pythoncomplete.vim Python
rubycomplete.vim Ruby
syntaxcomplete.vim from syntax highlighting
xmlcomplete.vim XML (uses files in the xml directory)