Microsoft Excel Callback DLL, possibly used for XLLs.
Two somewhat important exported functions of XLCALL32.DLL seem to be Excel4 and Excel4v. Then, there is also XlCallVer.
The functions Excel4 and Excel4v (as well as the functions Excel12 and Excel12v, which are not exported by XlCall32.lib) provide the functionality to make calls («call backs») into Excel.
Function suffixes
The function suffixes 12 and 12v have been introdued with Excel 2007 while the 4 and 4v suffix was already existent earlier.
The API version to be used is determined by the value of the macro XLL_VERSION (which can be defined to be 4 or 12).