Sampling units will vary depending on how the researcher makes meaning; they could be words, sentences, or paragraphs. In the mission statements project, the sampling unit was the mission statement.
Context units neither need be independent or separately describable. They may overlap and contain many recording units. Context units do, however, set physical limits on what kind of data you are trying to record. In the mission statements project, the context units are sentences. This was an arbitrary decision, and the context unit could just as easily have been paragraphs or entire statements of purpose.
Recording units, by contrast, are rarely defined in terms of physical boundaries. In the mission statements project, the recording unit was the idea(s) regarding the purpose of school found in the mission statements (e.g., develop responsible citizens or promote student self-worth). Thus a sentence that reads "The mission of Jason Lee school is to enhance students' social skills, develop responsible citizens, and foster emotional growth" could be coded in three separate recording units, with each idea belonging to only one category (Krippendorff, 1980).
If you want your research to be purely cognitive, you can use the above techniques, which are context-based and context-oriented solutions. But if you want your answer, along with other applied research, to eventually pave the way for a theorizing in the social sciences, I suggest you read this book as well.