They’re pretty much the same thing, but the state calls them slightly different names.
The Initial Report is just your “first” Biennial Report. The names, due dates, and filing fees are different, but the information they communicate to the state is the same.
Said another way, the Initial Report is just the Biennial Report with a different name and due date.
And while the Initial Report is a free, one-time filing, the Biennial Report costs $100 to file every two years.