The idea behind mounting piles is much the same as mounting a Unix directory to a mount point. All requests through the mount point will be passed the sub-pile mounted to that point and any routing (directory or page) information following the mount point navigates within the sub-pile.
Currently, MIC only supports one level of mounting. Their can be one primary pile and one level of children. Future versions of MIC may allow for sub-piles to mount sub-piles as well. Programatically, this is not that hard, but may induce coders to create hard-to-resolve errors.