Apple Final Cut Pro 6 - Manual (Page 1739)

256 Part III Output
 Crop: Scales and center-crops your clip or sequence video to fit in the clean aperture
dimensions of your exported movie.
 Fit within dimensions: Changes the shortest dimension of the exported movie
(usually the height) so that the source sequence or clip fits within the exported
movie dimensions when scaled.
For example, if your sequence dimensions are 1280 x 720 (16:9) and the exported
movie dimensions are 320 x 240 (4:3), the exported movie dimensions are changed
to 320 x 180 (16:9) and the exported sequence is scaled to these dimensions.
When the “Preserve aspect ratio using” checkbox is not selected, the sequence or clip
video is stretched to fit the export movie size.
Deinterlace Source Video
Select this checkbox to avoid artifacts when scaling interlaced video formats such as
standard definition NTSC (480i), PAL (576i), or 1080i video.
16:9 cropped
to 320 x 240 (4:3)
16:9 within
320 x 180 (16:9)