What does the INLINE hint do?

I have developers adding this to a lot of queries. But I cannot find an actual definition. It is not in the documented hints though it is listed as a workaround to numerous bugs..

The developers use this in combination WITH clauses

WITH view1 as (SELECT /*+ INLINE */ ... )

select /*+ INLINE */ from a where ...

I just need a description of what it actually does so I can explain to developers when it is appropriate and when it is not. I suspect they found a hammer and just throw it in whenever something is slow. Tables have billions of rows.

Thanks