The "none" parameter in CBC models is the utility of not choosing any of the concepts in a choice set. Its precise meaning depends on how exactly you've worded it, but a general "none" wording means that the respondent prefers not to have any of the concepts offered. In a simulation, which is the best way to interpret ANY of the utilities, when you allow "none" into the simulation, the % choosing it are those who choose not to choose any of the products you've specified. Again, these could be people who choose to stay out of the market, to purchase elsewhere, to purchase later, etc., depending on how you've worded the "none" alternative in your questionnaire.