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The PPI only matters if you are sizing an image to a particular print size. Say if you want an 8x10, and want it really high quality, then setting it to 300 PPI will produce a 2400x3000 pixel JPEG.
However, if you exporting a 12megapixel image as a 3000x4000 pixel JPG from Lightroom or Photoshop (or any other editor) then the PPI is irrelevant because you've already specified the output pixel dimensions. A pixel is a pixel is a pixel, it in itself has no PPI setting. You can do a test and see for yourself. Export an image as a JPG at particular resolution (say 800x1000 pixels) with a PPI of 10 (yes "ten") and with a PPI of 300. You'll see that the resulting file size is the same. A binary compare of them will only show a byte or two difference. The PPI at that point is just a piece of metadata which most online printers (Costco, Mpix, etc.) ignore as they only look at number of pixels. Each of those companies have their minimum resolution requirements for prints. In other words, a 800x1000 image with a PPI setting of 10 (ten) does not mean you can get a 80-inch by 100-inch print, you don't have enough pixels.
Hope that helps and doesn't confuse you.