ifile is a general paramter which can be used to break the parameter files
into sub parts,
For example u can have a standard init.ora parameter file for all instances and
another file containing specific paramters tuned to specific instances.
You can use ifile parameter in your general init.ora parameter file to point to this specific
instance paremer file.

Simillarly you can use ifile paremeter to break tnsnames.ora file into sub parts.