What is .fek file?

Hi,

I am wondering the purpose of .fek file?, When I create a feko geometry and save it. it does not create a .fek file.  I noticed that it creates when I run the feko solver. When I submit a job for a supercomputer for running simulation, it asks for .fek file and often give errors due to the problems related .fek file.  If someone can clarify this(at least the role of .fek file) that would be great.

Thanks in advanced