normal xml parsers know nothing about your specific processing instructions, it has to be your own application which processes your xml files to process them
agree with pzma, it is up to the application which processes your xml files, you can put anything in <?....?>:
<?proc exe="whatever.exe" paramcount="2" param1="a" param2="b" ?>
they are application specific, the application processing the xml file needs to understand them/process them or simply ignore them
for example, you have an applications which process xml files, in the input xml file, you specify
<?peppi SomeTag="SomeTag2" ?>
if the application understand this and it can change SomeTag to SomeTag2, if the application does not understand this processing instruction, it will igore it