How RMAN backs up blocks of datafile on certain SCN?

 Hi, can anyone explain me how RMAN backs up blocks of datafile on certain SCN?I've read that: RMAN does not require extra logging or backup mode because it knows the format of data blocks. RMAN is guaranteed not to back up fractured blocks. During an RMAN backup, a database server session reads each data block and checks whether it is fractured by comparing the block header and footer. Also, RMAN does not need to freeze the file header checkpoint because it knows the order in which the blocks will be read, which enables it to capture a known good checkpoint for the file.