If you are processing a lot of these, and if this item will be used for further processing, you could consider turning this into a class which might be easier to manage. This would also allow you to more easily customise your ratings.
I'm thinking maybe a factory might serve you well also which could instantiate and return your assessment entity. You could then leverage a dependency injection strategy for any other functionality.
Its a little hard to tell what you require this for without a bit more context.
If this is a once off, I would refactor to
assessmentEntity.RatingType = strArrIds[1].Contains("CO") ? String.Concat(long.Parse(strArrIds[1].Substring(strArrIds[1].Length - 1)).ToString(), "L") : "0N";
Assuming "0N" is some other default rating..