RSS, Atom and JSON Feed
Three formats for the same thing: a list of recent updates. As a reader, you almost never need to care which one a site uses.
The three formats
Sites publish their updates in one of three formats: RSS 2.0, Atom or JSON Feed. All three say the same thing — "here is what we posted recently" — in a different notation.
RSS 2.0 and Atom are XML; JSON Feed is JSON. RSS is the oldest and most widely used. Atom came later and tightened up the parts of RSS that were left ambiguous. JSON Feed is later still, made for implementations that would rather not deal with XML at all.
What the difference means for you
Readers are expected to handle all three, so there is no format to pick when you subscribe. This service reads all three.
The difference shows up when a feed leaves information out. Atom requires a unique ID per item; RSS 2.0 lets it be omitted. Without an ID, a reader has to guess whether two entries are the same article, usually from the URL — so a site changing how it builds URLs can make old articles reappear as new.
Dates work the same way. RSS 2.0 allows the date to be omitted and uses a different format from Atom. A feed with no dates can only be ordered by when it was fetched.
Full text is not a property of the format
There is no such thing as "Atom means full text". Whether a feed carries whole articles or one-line summaries is a site setting, not a format. Plenty of RSS 2.0 feeds carry full text and plenty carry almost nothing.
Public pages on this service show titles and links only, never article bodies. Reading happens in the reader or on the original site.
Podcast feeds
Podcasts are distributed over RSS too. The audio file is carried as an enclosure, with extra fields for things like the publisher and the running time.
You can subscribe to one in an RSS reader, but playback and resuming where you left off are better handled by a podcast app.
So which should you use
As a subscriber, you do not choose. Enter the URL of the site and whatever it publishes is detected automatically.
If you publish a feed of your own, whatever your tooling emits by default is fine. Readers handle all three.