Ordnance Survey Vector Datasets and GML Readers

Hi - I'm having some problems with translating the OS VectorMap Dataset - I know there's a reader for each of the OS Vector products - and I've cross posted on another thread (Issues Reading in GML)

 

BUT

 

what I need explaining to me are the different approaches that seem to have been adopted by each of the vector products, and the GML, and their Readers in FME.

 

 

I've had no problem at all in translating OS MasterMap, I've created quite a nice workspace to translate and style the data into MapInfo; nor have I any problems with the VectorMap Local.

 

But why does VectorMap District require to have an xsd called by a URI, and then store feature types in an SQLLite db (?)  and so on...

 

If anyone can enlighten me on the different approaches that seem to have been undertaken, then I would be very grateful !