Representation of structures is not very difficult, at least for electronic structure calculations. Several formats can be used, and code to convert between them exists (open babel, ASE,...). When stored in one of the standarized format it will be just a list of labels and positions (like xyz), cell and periodicity information.
AtomLabels is an evaluation value storing an array of strings (normally char 8). if not defined it is assumed it has the same value as the previous evaluation
AtomPositions is an an evaluation value that is an array of 3 positions. If not defined it is assumed it has the same value as the previous evaluation.
Adding topology information is currently not considered.