This is a good question as it is one of the more complex aspects of the spatial simulation. During a spatial simulation, ST-Sim iterates over each primary transition group to generate “events” for that primary transition group. Events can be a mosaic of transition types within the primary transition group that is being generated, therefore you should only combine types within a primary group that you would expect to be part of the same event. The reason you don’t want to include the same type in more than one primary group is that it will be simulated more than once (i.e., once for each primary group) and thus the input probabilities will not be reflected in the model output. Primary groups could be useful for combining other types of non-fire transitions. For example, you could have different level of severity for insect and disease transitions which could form a mosaic within the same outbreak “event”.