In the TeX (documentation) part of a section, ‘@|’ inserts a vertical bar. This is useful inside LaTeX verbatim environments. (A simple bar would signal a shift into code mode, which is probably not what one wants.) For an example, see ‘@|’: Literal vertical bar, or optional line break.
In a code part, ‘@|’ inserts an optional line break in an expression—e.g.,
‘f(a,b,@|c+d,...)’.
This helps TeX to break the line at an appropriate place. If the line does not need to be broken, the command does nothing. [Compare ‘@|’ with ‘@\’ (see ‘@\’: Force a line break, then indent) and ‘@/’ (see ‘@/’: Force a line break, preserving indentation.), which always break the line.]