If task records were created as part of simultaneous routing, they will have a letter (A-Z) followed by a dash (-) following by ANY or ALL to indicate whether the ANY or ALL simultaneous routing occurred. The initial letter (A-Z) is only to display the set of simultaneously routed records. A will be assigned to the first set it finds, B to the second, C to the 3rd etc.
If you undo task records that have been generated via simultaneous routing, they will be processed as a set.