Well, it is very simple. The owner can choose the number of weeks he can use for personal use, the number of weeks he can use for rental and the number of weeks he can use for exchange. And as the person chooses other units in other resorts, also others have the right to use his. So, sometimes, maybe the period in which the person is traveling, there is someone using his unit. And all these things can be planned one year earlier and the owner who determines the usage of his unit and the period.
And if someone wants to use all the weeks for exchange, then of course he neither can come to stay in his unit nor rent it out and consequently he would not generate any rental income on that year as long as he reserves all the weeks for traveling. Besides, the owner can modify the usage of weeks each year.
Yes, I assume the points are valid for accommodation alone but I can check with an expert and get back to you.