In the context of subtitling, transcoding refers to the process of taking subtitle data in one format and recoding it into another format. This can be considered a special case of pass-through subtitling. See the Pass-through White Paper for a more general overview.
Subtitle Transcoding is often required where a service is first broadcast with subtitles in one format but downstream is re-broadcast and requires subtitles in a different format. A common example is where analogue services are simulcast as digital services. The analogue services require subtitles in Teletext format but the digital service requires DVB subtitles. In this case the Teletext subtitles are transcoded to DVB. The delay in the signal due to the video MPEG encoding can be used to allow time for the subtitle encoding so the presentation timing of the DVB subtitles closely matches that of the original Teletext subtitles.