I won't say more than what you can find here:
Export to IFC - Synchronization Wiki - iTwin Services - Bentley Communities
You can also search the Blog for mapping file (json):
(+) iTwin Services Blog - iTwin Services - Bentley Communities
The latest version (4) is taking into account corridor classification but with IGNORECLASS so it won't export with this mapping file unless you change it and assign an ifc class.