That's the width/height ratio of a pixel. If your square-pixel (1.0 PAR) still image isn't interpreted correctly for a non-square (.9 PAR) project, the image may appear squeezed. See
http://livedocs.adobe.com/en_US/PremierePro/3.0/WSAB9F51A7-8D3F-40db-A512-F...
You should be able to correct the distortion of a single still, by right-clicking it in the Project panel, then selecting Interpret Footage. In the Pixel Aspect Ratio pane, select Conform To, and select the pixel aspect ratio that matches that of your project.
Steve Muratore
Sr. Technical Writer
Adobe Systems, Inc.