In the series table itself seriesID is the tv.com ID. But you don't have to mention that here as that brilliant piece of naming isn't reflected in the api. seriesID is only the tv.com ID when in the series table. In other tables seriesID matchs ID in our series table.
Only me and you have to worry about that. Or anyone trying to work with our DB dumps.
ID never used to be a field on GetSeries. It's not even mentioned in our documentation. I'm not sure when it got added but as far as I know it really shouldn't be there. I'd be reluctant to remove it though as many people are likely using it.