archive_active file.

I recently built a postgresql 12 server and forgot to turn on WAL archiving. I modified my postgresql.conf file and stopped/started the server. After a day and many WAL files being generated archiving did not start until I found a suggestion to create and empty archive_active file in my archived WAL segments directory. When I did this archiving began. Does anyone know why archiving didn't start after the stop and restart and can anyone tell me that the archive_active file is a specific postgresql file or is it just that the archived WAL segments location has to have a file in it before archiving begins?

Thanks

Craig A.