Reading PyDev documentation you I have found this:
"The Forced builtin libs are the libraries that are built-in the
interpreter, such as __builtin__, sha, etc or libraries that should
forcefully analyzed through shell introspection (the other option to
analyze modules with too much runtime information is providing
What does "shell introspection" mean? And what do you think the writer
want to say with "too much runtime information"?
Shell introspection means that PyDev will spawn a shell, import the module and do a dir() at them provide completions based on that.
The Predefined Completions is for the use-cases where that doesn't work or doesn't provide enough information.