from vim's documentation on `:mksession` (:help :mksession):

 ...

 10. If a file exists with the same name as the Session file, but ending
     in "x.vim" (for eXtra), executes that as well.  You can use *x.vim
     files to specify additional settings and actions associated with a
     given Session, such as creating menu items in the GUI version.

we already have Session.vim ignored.  the Sessionx.vim file, like
Session.vim, is a user file.  a user would generally want that file kept
private or for themselves, and the public or a team fetching from or
sharing the repository generally have no interest in a file relevant
only to a particular user.  so it's a good idea to get git to help us
avoid mistakenly sharing the file.

While will never be generated by the vim source code.
http://unix.stackexchange.com/questions/326707/vim-what-are-all-the-possible-swapfile-extensions

Added auto-generated tag files.

Anything matched by `*.un~` will also be matched by the last rule, `*~`.

The old pattern was too broad and was making VIM ignore Scheme (*.scm) files.
Now it only matches files that start with . or _.

.*.s[a-w][a-z] matches named swap files.
.s[a-w][a-z] matches unnamed swap files.

Vim creates buffer swapfiles starting at .swp for buffers that are
active but untitled and unsaved.