So this is not officialy documented any where but from what I have managed to gather is that the "number" is divided into five parts. Each part belongs to a shard. for example, US,EU,JP...
Each number is actually the number up updated "public and/or private" item changes recorded on the shard.
So shard as some people like to call it has seen 97million and a bit item changes.
You can actually just get data from each shard by setting the other shard numbers at extremly high numbers.
will return a set of stashes and items ONLY belonging to shard and the next change ID is
Which means THIS request returned 2542 items from the start of the game, and on that next ID you get the next DELTA SET of data.
This subset managed to return 2364 items for shard (4888-2524) which is a similar data set size.
Now the normal API returns a number that looks a bit strange and is hard to follow because the API fills a buffer of about 5mb of uncompressed data, then calculates the new number just for your call, based on all the data it could get from ALL the shards. You then need to follow your change id's get the next set of data in sequence.
If you call a number that is over the amount of items seen on all shard you just get an empty response, with that change ID and no items. This is possible, if you can manage to catch up to real time.
But that is only possible if you do some clever maths your self.
And the maths is what makes sites like poe.trade able to det data indexed within seconds and is their "secret sauce"