Followups are to HD Ticketss what comments are to usual drupal nodes.
Once a HD Tickets has been created by a USER or an ATTENDANT, followups can be added to it and threaded. Followups carry status values, and the value of the latest followup to a ticket (if any), defined the current status for the HD Tickets.
In the current implementation, they are implemented as nodes themselves, but it might be best to implement them as comments instead to take advantage of the builtin thread mechanism.
Followups always charge against a HD Contracts, decrementing the contract's counters according to its current charging mode.
Only TECHs can issue followups to incrementing (instead of decrementing) HD Ticketss, which correspond to contract extensions.