What is the use of the command \IeC?

Question

It is produced in streams by newfile when I use special characters.

Answer 1

Update

Since some years, LaTeX no longer writes \IeC{...} in the .aux files, but the actual UTF-8 bytes. So the problem has essentially disappeared.

Original answer

When dealing with UTF-8 coded document, one must keep in mind that characters are encoded by sequences of one to four bytes and that traditional TeX engines (including pdftex) are not able to directly interpret multibyte characters.

Thus multibyte clusters are, as soon as possible, translated to the LaTeX Internal Character Representation. This is feasible because in a multibyte cluster the first byte tells about the length of the cluster and so it's not too difficult to compute from this and the following bytes the Unicode point we are faced with; LaTeX maintains a table mapping Unicode points to LaTeX commands. For example, the multibyte cluster that the screen shows as Ü gets translated into \IeC{\"U}.

What's the purpose of \IeC? When \"U is found by LaTeX, it tries to expand \", which is wrong when writing to an auxiliary file: we want that \chapter{Über} writes something equivalent to Über in the .toc file and not the complicated sequence of commands needed to print it.

Thus \IeC: it does nothing during normal typesetting, but when LaTeX is writing to auxiliary files the tokens \IeC{\"U} are written out literally.

If you want to write literally to your own auxiliary files, then this mechanism must be disabled, which can be obtained by protecting the tokens with \unexpanded. An example of what you want to obtain should be shown.