I am uncertain about the actual question. If you are developing a plug-in or otherwise interfacing with the guts of ID, I suggest asking the question in the SDK forum:
Otherwise, what is it you are wanting to know concerning importing XML into InDesign?
I would like to read the xml file for getting some path parameter and some user details. So its more of a paramter file for my plugin, which I need to parse in my C++ code in the SDK.
Pre load or post load?
Post load, it can be parsed via scripting.
Pre load, it is simply text, likely UTF-8. It can be read via c++ programming either line by line or loaded into an array if the file is large and the data you want is not in the first X number of lines.