DASH manifest. There is one period, it has a template that is divided into segments, which we download. What do periods mean?

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1. What are these periods in general and in what cases can there be more than one?
2. If there are several periods, then you need to take the duration of the previous period (add all segments) and add it to the start time of the new period, and each segment will already be calculated by the formula: segment time + duration of all segments of the previous period?
I read the docks, there is nothing at all clear, except how to add the time of the periods, if it is set, but without examples of use, it is generally not clear what to do with it. Personally, I have seen a manifest with only one period, but I'm not sure that a manifest with several periods will not arrive someday.
That is, the essence of the question is how to name the segments from each subsequent period, but I cannot name them the same way as the segments of the previous period.