In traditional hot backup, first needs to switch tablespace to backup mode.
Alter Tablespace users Begin Backup;
It'll freeze SCN in the file header of corresponding data files, while SCN of data blocks continute increased to reflect the changes. When recover the database, these two SCN will be compared to see if data block needs to be recovered from archived log files.
RMAN writes frozen SCN of data file header into either control file (nocatalog) or recovery catalog instead to "Alter Tablespace users Begin Backup;".