What does \errorcontextlines do?

In the `rotating` package examples there is a command included before the document class.

``````\errorcontextlines32
\documentclass[twoside]{report}
``````

When I apply `\meaning\errorcontextlines` to it, it does not show any definitions. What does it do?

-

`\errorcontextlines` refers to the number of lines output by TeX upon finding an error, which should provide some context for the user in terms of the why the error occurred.
If you use TeX format packages designed by others, your error messages may involve many inscrutable two-line levels of macro context. By setting `\errorcontextlines=0` at the beginning of your file, you can reduce the amount of information that is reported; TeX will show only the top and bottom pairs of context lines together with up to `\errorcontextlines` additional two-line items. (If anything has thereby been omitted, you'll also see '`...`'.) Chances are good that you can spot the source of an error even when most of a large context has been suppressed; if not, you can say '`I\errorcontextlines=100\oops`' and try again. (That will usually give you an undefined control sequence error and plenty of context.) Plain TeX sets `\errorcontextlines=5`.
In the above discussion it is assumed that `\oops` is undefined.