Doubt regarding IRP_MJ_ACQUIRE_FOR_CC_FLUSH

Hi,
I am developing a minifilter for encryption/decryption. I wrote some code
which encrypts data in pre operation of IRP_MJ_WRITE.
But I noticed that apart from files/directories, the write function is
called many times by IRP_MJ_ACQUIRE_FOR_CC_FLUSH and
IRP_MJ_RELESE_FOR_CC_FLUSH.
When are these get called??
And what am I supposed to do to encrypt file data only??

thanks in advance…


Kapil Bhadke