poser in Python speak, is a module.
It is built-in to the Poser executable, so you can only use the Classes and Objects defined by from within Poser.

Scripts can go anywhere you like, but must be run from within Poser if they use anything from the poser module.
If a script refers to other modules (script or compiled) they must be in a directory on the PYTHONPATH, or in an appropriate directory in the Python library.