This sounds like an OCP question. . . .
Yes, RMAN creates separate backup sets for each group of tablespace block sizes. Suppose your default block size is 8K and and you also have one other tablespace defined with a 16K block size. RMAN will create a minimum of two backup sets, one for the 8K tablespaces and one for the 16K tablespace.
Think about it this way. RMAN is copying the blocks belonging to a tablespace to the backup set. Which is easier to manage, ensuring the backup set contains blocks all of the same size or being able to mix block sizes within the backup set? RMAN places information in the beginning of the backup set that all blocks found in the set are a certain size. On recovery, RMAN can proceed block by block through the backup set. If you allowed mix block sizes in the backup set, then RMAN would have to go through a process of determining how big the next block in the backup set is. It is much easier to ensure that each block is the same size in the backupset. This may require multiple backup sets when your database is small enough to fit into one, but you're letting RMAN manage the recovery for you, so you shouldn't care how many backup sets RMAN chooses to create.
This was first published in April 2006