The single file transcoding is used to transcode your source file to a single quality. Suppose your source file is 1080p, you need to transcode it to 720p then you can use this transcoding method. Click here for more information
The combined file transcoding is used to transcode your source to different qualities. This method is mainly used for ABR streaming. Suppose your source file is 1080p and you need to switch the quality to 1080p,720p, and 480p based on viewers’ internet speed then you can use this method. You can use SMIL based transcoded URL for the delivery. Click here for more information.