I work in the medical field and work specifically in the coding department. NEC (not elsewhere classified) and NOS are NOT NOT NOT NOT NOT synonyms. NEC is where the physician documents something but it is not found in a coding book therefor it is a specified term, not listed. NOS(not otherwise specified) is a generic term when not enough specificy is given to select a specific type of diagnosis. Ex. a dr documents a fracture. (NOS). then a dr says manubrium fracture of the wrist (NEC) because there isnt a code in the codebook for that diagnosis. Make sense? I have a 4 year degree in this field.

So onto the coding part.

296.66 Bipolar I disorder, most recent episode (or current) mixed, in full remission
I'm assuming they mean your most recent episode was mixed (mania and depression within the same episode) and now it has resolved and you have no current symptoms AT THE MOMENT. This can change as we all know with bipolar.
a quick google search says this about mixed episodes "
A period during which symptoms of a manic and a depressive episode are present at the same time. People who experience mixed states describe feeling activated and “revved up,” but also full of anguish and despair. Rapid, pressured speech can co-exist with impulsive, out-of-control thoughts of suicide and self-destruction or aggression. Hopelessness, irritability, uncontrollable swings between racing thoughts and a feeling of “being in blackness” can all happen over the course of minutes."

296.89 is for Bipolar II. it is an unspecified (NOS) code. I wouldn't pay too much attention to this code since you have a more specific diagnosis with the other code. This is just a provider being lazy and not selecting the right code. You can ask for a copy of your medical records and ask that this diagnosis be taken off of there because it is incorrect. You can also tell the office that they incorrectly billed your insurance for the wrong diagnosis. I am also a medical record auditor and we look at the documentation to make sure if supports the code assignment so i'm sure on this case we would be changing that code to another bipolar I code.

296.63 Bipolar I disorder, most recent episode (or current) mixed, severe, without mention of psychotic behavior
This is basically the same as the first code but the only difference is it is severe and without psychotic behavior. There is a code that you can select for psychotic behavior.

If you have any other questions don't hesitate to ask