I think the best way to explain this is with an example:
Let's say we have a Connectivity and we launched a simulation. We export the resulted TimeSeries H5 and then we deleted both the TimeSeries and the Connectivity. After that, we wish to reimport the exported TimeSeriesH5 while we don't have the connectivity that was used for launching because we just deleted it. This should not be possible, but unfortunately it is.
In TVB 1.5.8 TVB gave an error when such scenarios where happening and it should be the same now for all datatypes that have dependencies.