What does "Posts older than 365 days will be removed." mean exactly?

A lot communities have "Posts older than 365 days will be removed." in their description, but what does this mean exactly? It never says.

• The database of posts only goes as far as 1 year, everything older gets wiped? I hope and assume that's not it.
• Posts with links to articles older than a year get removed?
• If a thread is over a year old, no new comments will be added?