Allow multiple task levels to represent parallel tasks
"When working with many clients their story contains events which happen in parallel across multiple users. Currently, the SOB map only allows you to have a single linear set of 'Tasks'.
By having multiple 'task' tracks (one per user group) would allow tasks to happen in parallel; this would also allow, for example, things like a 'system' track where automated tasks happen. It would potentially also allow for User Persona's to be attached to the 'track' - I've found the groups to identify with the tasks much more quickly when they can see who's it's related to.
This is touched up in Jeff Patterns 'User Story Mapping' book (page 85 - 1. User Story Mapping for Large Products)" - Neal