Mmm, interesting, young Sir - if indeed you are a Sir.
You can apply for 4 blocks of 5 days leave. After these has been allocated you can apply for an additional 5 days leave called 'guaranteed days off'. You must take them as a block of 5 days and they will be recovered from your normal pattern. In other words, you will get your roster for a particular month and find that one week you are working 6 days instead of 5. There will be 5 such suprises until these RDOs are recovered. So, you only really get 20 days leave which you can book and call your own.
FQ