when you have two different stays (check-in/check-out, no re-check-in at same place at same day; regardless of number of nights) you will get a certificate to stay a night for free (limited to cat. 5 I believe)
tip: if you want to use this in a multiple nights stay; book only one night (at payment the certificate will appear)
book the remainder of the nights then to start/end with the "free"/certificate-night
Ramoneur is right. I will add that to be able to redeem a certificate...you must choose "use rewards points" when booking a room or the certificate will not show up when getting ready to finalize payment in my experience. Yes it is category 1-5 hotels. I have earned my megabonus already!!!