When does diff use | in a side by side diff?

[Sigbert Klinke]

Hi,

in the documentation to diff is said under "Showing Differences Side by Side":

‘|’
The corresponding lines differ, and they are either both complete or
both incomplete.

What does _complete_ and _incomplete_ exactly mean, just newline is missing or 
not?
How is _differ_ defined?

See also the examples in

*https://askubuntu.com/questions/1302054/when-does-diff-use-in-a-side-by-side-diff
  and

*https://askubuntu.com/questions/1105189/diff-side-by-side-output-with-incomplete-sign

Thanks in advance

Sigbert