The
sliver in the top is a flaw. Note that the grain deflects on either side of the mark. In my experience it is the remains of a twig around which the tree grew. Trees grow in girth, "swallowing" objects once on the surface, including their own branches, with later wood. Knots are simply the cross-sections of branches which have been surrounded with wood.

If the line isn't the actual wood of that twig, then it is some wood filler which has been placed to fill the void left by the missing twig. It looks to me like the twig itself, though.

While I would describe it as a flaw, many people prize such unique marks in wood. Because of the deflected grain on either side, it is clearly not a gouge or other damage made after the wood was turned.