Split face means a white blaze that goes up over the forehead which divides the face into right and left halves. Most JA are born with blazes but they go away in time. Some do not fully fade, however, and on a red it can look bad. On brindles, it depends on the dog's other markings if it "suits" the dog. If it doesn't go past the ears it may be fine. Connecting to the neck/collar is always bad however.
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EDIT: Illustration of blaze fading over time.