For a full description of the delivery requirements, see the File format section. In general, the BBC accepts EBU-TT part 1 with STL embedded for broadcast, and EBU-TT-D for online only content. There are many formats in circulation for subtitle files. Typically delivered as a separate file, closed subtitles can be switched off by the user and are not 'burnt in' to the image. This document describes 'closed' subtitles only, also known as 'closed captions'. The majority of these viewers are not hard of hearing. Subtitles are primarily intended to serve viewers with loss of hearing, but they are used by a wide range of people: around 10% of broadcast viewers use subtitles regularly, increasing to 35% for some online content. If you are new to subtitling, please start there.