I would like to download year by year planet files for a map project I’m working on but I don’t know which file or files in the indexes like the ones you see listed in the link would be necessary for me to download to run through mapmaking software. What purpose do each of the different planet, changeset & discussion files serve?
Here’s a bit of an explanation You likely want a “planet” file (with OSM data in it) not changesets or discussions (which are just metadata).
Why are there a lot of different planet files for the same year?
Data is added all the time, so an equally valid question might be “why is there only one a week?”.
Apologies if I’m misunderstanding where your question is coming from…
So all these planet files are for every week of the year?
That’s the intention, although historically there have been times when the time to generate took too long, resulting in delays.
It’s worth mentioning most people don’t need to go near a planet file because regional extracts exist, and also that if you are interested in historical data you might be interested in a planet history file, which has not just all** the data but the history up to that point too. Regional history extracts also exist.