The -15, -16, and -17 are different flux systems that coat the electrode.
The -15 is a limestone based flux similar to the coating found on a low hydrogen electrode. It is a fast freeze electrode suitable for all positions, it produces a pronounced convex weld bead, and has the "best" penetration of the three flux systems mentioned. Incomplete fusion in the root of a fillet weld can be a problem if the welder doesn't angle the electrode properly.
The -16 is a titania based flux that produces a flatter bead, has less penetration which means you most likely will not get fusion to the root of a fillet weld, and it has a more fluid weld puddle that makes it difficult to use in positions other than flat or horizontal.
The -17 used to be included with the -16 coatings, but was made a separate classification a few years ago. It is a titania based flux system with higher silicon content that produces a flat to concave weld bead, very fluid puddle, shallow penetration, fine closely spaced ripples on the surface of the weld bead, and can not be used in positions other than flat and possibly horizontal. Incomplete fusion at the root of a fillet can be expected.
Best regards - Al
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