There are two scenarios in which TTML samples, i.e. short snippets bounded in time, are needed, which collectively relate to a single entity.
The first is for live authoring, in which an authoring station may need to create and emit a series of short documents for display with low latency. The second is for distribution, in which an encoder or packager may need to divide a longer document into a series of short documents for managed data rate distribution to an onward chain. An example of this would include use in MPEG DASH.
In the live case there is a follow-on use case, which is to permit an archival device to accumulate a set of TTML documents together to form a larger one, i.e. one whose temporal extent is the union of the temporal extents of the input documents.
In the distribution use case there is also a follow-on use case, which is to provide direction to presentation processors to enable them to display the series of documents with no visible join between them, i.e. as though they were received as a single document. There may be a need in this case to provide some guidance, either in the specification or in the contents of the provided documents, to minimise the implementation cost (at design time and at run time) associated with this use case.
At first glance TTML can already handle these scenarios by simply creating small documents. However there is no support for chained related documents. For example the use of xml:id expresses a uniqueness requirement that is per-document not per-set-of-documents.
A starter document for this (including errors) was posted to the reflector at 1.