Prewound bobbins size for the Janome 15000

TC Wiley

I have had my 15000 for 5 years and I love it. I have been using size L pre wound bobbins for embroidery and have been pretty lucky. Occasionally a bobbin acts up and that one goes in the garbage. I recently learned that I should have been using size 15 pre wound bobbins. Is that what y’all use? What is the difference between size 15 and size L?

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