It indicates the attribute of the IPE file when it is used in the reloading.
If the value is 'primary', the packages in the IPE file will be the
first packages in the next reloading procedure.
If the value is 'secondary', the package in the IPE file will be used
if the primary packages fails.
If the value is 'none', the IPE file is only unpacked,
will not be used in the reloading procedure.