For some reason on some of my photos there is this 'J' icon:
What does it mean?
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When you shoot RAW + JPEG the Photos app stacks the two images together. The J means you have a stack with a JPEG file and a RAW file and the JPEG file is used as original. To change to the RAW file go into edit mode (enter) and choose Image - > Use Raw as original. The letter will now change to a R.