Bobbin holders/cases


I've been reading up on the 12000/15000 and read that there are 3 bobbin holders/cases (why do Janome call them holders - before now I have always known them as cases?)My question is why are there 3 holders?  Is it not possible to adjust the tension on just one holder?  I have never had to have a separate holder/case on any of my previous machines over the last 50 years and if a holder does have an adjustment screw could one not just adjust it?Is it in order to just save time when swapping from one function to another, or to assist those that have issues changing tensions. In fact I don't actually really ever need to change the bottom tension - just the top - so I would really like to understand what makes these machines so special that it seems they need 3 different set holders.Many thanks for any explanations.
Anne in Stubbington, UK
With all its sham, drudgery, and broken dreams, it is still a beautiful world. Be cheerful. Strive to be happy.  "Desiderata" Max Ehrmann

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