Yeah, the only difference I can see from the specs are that ndjson SHOULD be served with the type `application/x-ndjson` while jsonl does not seem to specify a mediatype.
I was curious whether .jsonl or .ndjson is used more often. Based on frequency in GitHub it looks like both are popular but .jsonl enjoys slightly wider usage:
> This page describes the JSON Lines text format, also called newline-delimited JSON.
And then I saw this at the bottom of the ndjson.org page:
> Site forked from jsonlines.org
So is this a fork of the site only, or a fork of the jsonlines standard?