Understanding the data now depends not just on having a schema to be found in a registry, but your schema registry, with the schemata registered in the same specific order you registered them in. If you want to port some data from prod back to staging, you need to rewrite the IDs. If you merge with some other company using serial IDs and want to share data, you need to rewrite the IDs. Etc.
They're saying that for a sequential number, you must have a central authority managing that number. If two environments have different authorities, they have a different mapping of these sequential numbers, so now they can't share data.
Oh. Like, if you have two different schema registries that you simultaneously deploy new schemas to while also wanting to synchronize the schemas across them.
