The T and Cs suggest that the difference is between miles gained since the last flight (which can be extended by non-flying activity) and miles gained up to the last flight (whether by flying or otherwise), which can only be renewed with flying activity. If the experience of my partner (who flies AF/KL once in a blue moon and is therefore faced by the renewal issue every now and then) is anything to go by, it seems that FB send an email a few months before the expiry of miles stating the number of miles that are due to expire. In any event, it would be preferable for the discussion to be continued in the most recent thread devoted to the question on which there already have been a number of answers rather than starting a new thread every time.

The problem that you are likely to encounter in asking those questions is that probably 99% of the people on this board will not have been faced by the issue, not least because most are regular or semi-regular flyers and therefore will struggle to give you a definitive answer.