I'm curious about how eTicket numbers work as it seems to be always discussed rather vaguely.

I've sometimes had itineraries composed of multiple consecutive eticket numbers. What causes this to happen? Is it a straight-up "different fare components have different eticket numbers" rule? What I've found confusing in the past is that return-trips seem to all have the same eticket number, which leads me to guess that it's the fare component that matters.

One of my booked itineraries currently (rather strangely) shows the eticket number of flights earlier on in the itinerary as "after" the eticket number of flights later on in the itinerary. In other words, the eticket number of the first set of flights are "n+1" whereas the eticket number of the last set of flights are "n". Would this pose any problems? Is there a distinction between eticket numbers and "coupons"? (I recall frequently reading on fare rules "coupons must be used in order" or words to that effect). There was a schedule change on the itinerary, if that's relevant, though I can't remember if the ticket numbers switched when the schedule change happened or if they were always that way.

Hoping someone with a deeper understanding of etickets can shed some light.