Documentation for the API exposed from Pressflow Reading How do Pressflow Drupal sites handle user Cookies? I discovered that Pressflow 6 has a function that is not present in Drupal 6, but that is defined in Drupal 7; although, the code used by Pressflow 6 is different from the code used by the homonym function in Drupal 7.

Should developers use only the functions that are defined from the equivalent Drupal version (e.g. Drupal 6 for Pressflow 6)?
Pressflow is designed to be API compatible with Drupal. As a consequence you should only use the API functions of Drupal 6 when developing for Pressflow 6.