In the higher chanmodes, the modeling elements describe properties of the model that hsolve has to compute. If you change one or more properties - field values in the modeling elements - hsolve has to be notified of the change. This defines a data stream from the modeling elements to hsolve's internal data structures.
On the other hand, sometimes you will be interested in the many variables computed by hsolve. In that case hsolve is able to store all the computed variables back in the appropriate fields of the modeling elements. This defines a data stream from hsolve's internal data structures to the modeling elements.
These two data streams are not physically available, but parts of them are implemented via the actions explained in the following sections.