The ticket number is split into 3 parts.
First 3 numbers - IATA Plate . .
Next 3 numbers - Virtual block allocation . Not sure what else to call it - it's a carry over from the paper ticket days. It was like you would get a ticket series 2442 then issue the rest sequentially from the pre-printed paper sequence you have such as 100001 through 100999. One IATA agent will have a series of tickets issued which this number is always the same - it's like your block but without a plate - such that the plate is assigned when the ticket is "printed". Remember e-tickets exactly work like a virtual paper ticket.
Next 6 - Sequentially issued ticket numbers- Here - they are sequential from 000001 to 999999. When an agent plates a ticket, the sequence of the 2nd and 3rd blocks are consecutive with only the Plate number changing. i.e.

An agent might be issued all tickets in block 2190 and sequence 100001-199999.
If the first ticket they issue is on Air Canada it would be 014 2190 100001
If the second ticket was on Swiss it would be 724 2190 100002
and so on.
All the airlines in the world are sharing the 4 numbers in the middle such that no matter what you don't get a duplicate. However I believe some series are reserved for airline issue. I know when AC started e-tickets around 1996 I think they had the block 2190 assigned to themselves. But I think perhaps all the airlines could also use 2190 because they would never plate for someone else - only an IATA agent would plate for many. Not sure if that makes sense or not. i.e both AC and AA could have series 2190 because they would never plate for each other. i.e. 014 2190 100001 or 001 2910 100001 - - however - an IATA agent is given their sequence that is split between all the agents in the world. This sequencing needs to somehow be coordinated across all the airlines so you don't get duplicate ticket numbers issued.