This message appears when the file is created in one version of Poser and then opened in another (lower) version. Like opening a file created in ProPack in P4, or opening a P5 file in P4. Usually, this is no problem at all. You can get rid of the message by editing the file in question in a text editor. Near the top of the text, you will find the information which Poser version it was created in. Change it to your current version and re-save the file.