The sequence numbers are designed so that the hierarchy follows from simply listing the postings in sequence. 4 is the most recent main posting. 10 is more recent than 5 and has no followups. 3's, 1's, and finally 2's. We could assign ascending sequence numbers, but we actually assign descending numbers.
Insertion can occur at any arbitrary point in the hierarchy.
We cannot know ahead of time how many postings will be placed at any given location. Finally, we need to allow for a reasonably large number of levels of followup. In actual practice it is not unusual to see 20 levels of followup. That is, one main posting could have a followup which could have a followup which could have a followup, and so on, to 20 levels of followup. To accommodate these constraints, we have devised the following scheme for sequence numbering.