Forms of light scatter in an eyepiece:
1) near on-axis scatter. This is when a small bright star has a strong glow around it. It is hard to discover whether this is the atmosphere, dust/oils on the optics, rough surfaces on the lenses,
bad coatings, a large number of lenses, or just what, except by comparison with another eyepiece.
2) Ghosting. This is when a fainter "ghost" of the bright object drifts into the field from the opposite side that the bright object drifts in, moving in the opposite direction, and crosses the bright object in the center of the field, leaving the opposite side of the field when the bright object exits the field.
3) Edge of field brightening (EOFB). This might have various causes, including the distortion type in the eyepiece (all eyepieces have distortion) or bright spacers and lens edges.
4) general field scatter. We see this as a reduction in contrast, or as a scatter of light from the Moon into the black space beside it. It can determine how easily you can see some faint objects or details in objects as it relates to contrast. The predominant causes are poor baffling and blackening internally and/or rough lens surface polish on the lenses, or the level of coatings applied (fully multi-coated works best to suppress this).
5) Glow from outside the field. If a bright star is just outside the field, and you can see a glow in the field of the eyepiece telling you there is a bright object outside the field, there is someplace in the eyepiece light is not being properly suppressed. It could be all the previously mentioned issues, but it can also be reflection from baffles and eyepiece barrels if not adequately blackened.
6) Glare from outside the field. This can be a bright loop or multiple spikes or a combination of bright lines and glow. If the bright star or object outside the field causes this, it is usually a sign you are getting a strong reflection somewhere inside the eyepiece, and the only cure is to dismantle the eyepiece and blacken everything internal that can scatter light (except the lens surfaces themselves, of course, LOL).
IMO, #2, #3, #6 are design flaws. #1, #4, #5 can be improved with better execution (read higher cost) of the design.
To date, however, I have not seen 1 eyepiece that completely passes every test. #5 seems to be the hardest one to completely eliminate.