This works because the DNS RR types are numeric (represented as numbers) in the DNS protocol, whereas they’re displayed to human beings as textual values that are defined by Internet standards. For example, a query or response related to a host address (
A) record is represented in the conversation between computers as the number
1. New ones can be added over time, but old software doesn’t know aobut them.
The complete current list is
Maybe someday it will include other new ones that we don’t know about yet.