PASS THRU mode is a feature in iVI which can dramatically reduce conversion times. When PASS THRU mode is turned on on the Conversion Tab of iVI's preferences then a new column is shown in the main iVI window. This column can have a value of NO or YES.
YES means that the video will be copied (rather than converted) between the source and destination file dramatically speeding up the conversion process.
NO means that the video will be converted as normal. Under NO is a short message explaining why PASS THRU mode was't appropriate for this video file.
The messages are -
File Size The source file is larger than the maximum defined size for PASS THRU on the Conversion Tab of iVI's preference
MB/s The source file has a higher bitrate than the maximum set for the selected devices on the Devices tab of Preferences
Ref Frms The source file is encoded with more References Frames than are supported for the selected devices on the Devices tab of Preferences
fps The source file is encoded with more frames per second than are supported for the selected devices on the Devices tab of Preferences
Unkn Ref The source file has an unknown number of Reference Frames and so suitability for PASS THRU can't be determined
Not h.264 The file isn't encoded in h.264 and so may not be compatible with Apple devices.
HD/SD Format PASS THRU is currently disabled for HD/SD conversions.
Width or Height The source file has a resolution which is too large for the selected devices on the Devices tab of Preferences
ATV1:(reason) The source file isn't supported by the Apple TV 1 which is selected as a device on Preferences>Devices. The encoding issue is also listed.