What is a "modern" Spotlight impor… | Apple Developer Forums

What is a "modern" Spotlight importer

I noticed that mdimport has gained a few new options that refer to "modern importers":

Code Block  -e            Print all the modern importers available
 -m            Import files using modern importers. Specify the file type via -y and then the file url(s) via -u (separated by space) 
 -y type       Type of the file. Requires -m
 -u urls       URLs for the files. Requires -m

Code Block
-e Print all the modern importers available

-m Import files using modern importers. Specify the file type via -y and then the file url(s) via -u (separated by space)

-y type Type of the file. Requires -m

-u urls URLs for the files. Requires -m

mdimport -e replies with:

Code Block Extensions: id(501)

Code Block
Extensions: id(501)

I can't figure out what "modern importers" are.