The new deb822 format replaces the legacy one-line style. Hence, if you are trying to migrate to the new format, you are expected to comment all lines (by prepending them with # symbol) in your legacy .list file (or remove the file completely) and create new .sources file with the necessary sources in the new style.

APT reads files in the alphabetical order (earlier entries are preferred). Hence, if you want the file to be read first, name it as /etc/apt/sources.list.d/00-source.source, for example.

Withing a single file, sources specified on top of the file are preferred over the following sources.

The new deb822 format replaces the legacy one-line style. Hence, if you are trying to migrate to the new format, you are expected to comment all lines (by prepending them with the # symbol) in your legacy .list file (or remove the file completely) and create a new .sources file with the necessary sources in the new style.

APT reads files in the alphabetical order (earlier entries are preferred). Hence, if you want the file to be read first, name it as /etc/apt/sources.list.d/00-source.source, for example.

Within a single file, sources specified on top of the file are preferred over the following sources.