Main point is that
OPEN_CURSORSparameters limits the number of currently opened cursors by each session. If this number is exceeded the session will get en error. And
session_cached_cursorsis something totally different and independent from open_cursors. The idea behind the cursor cache is to hold "popular" cursors opened. If a cursor is being parsed every time it's opened, it causes a latch contention and concurrency starts to suffer. So to protect the database from a badly written code, you can cache the cursors. The number of cached cursors is set by the session_cached_cursors. The value is per-session. It can be set system-wide (needs a restart) or by ALTER SESSION. As I have understood, cursor cache is a part of PGA, so it will not effect the shared pool if this parameter is increased.
