Backup & Recovery :: Understanding FRA Files

I have been experimenting with RMAN over the past few days and have a fairly strong grasp on it. I setup a test scenario using a three tier incremental approach as follows:

Every four weeks take level 0
Every 7 days take level 1
Every day take level 2

Obviously I'm only taking one per day, so on days I take a level 0, I'm not taking a level 1 or 2. My scripts look similar to the following, with change to incremental level number.

rman target / <<EOF
run {
configure device type disk parallelism 4 backup type to compressed backupset;
configure default device type to disk;
configure backup optimization off;
configure controlfile autobackup on;
configure retention policy to recovery window of 4 days;
[code]......

What I'm trying to understand is what files Oracle is creating in the flash recovery area. I have several sets of files:

In the root level of the FRA, I see several files per incremental backup with names such as aambg4sa_1_1, aqmbg5mj_1_1, b6mbg614_1_1, etc. I was under the impression that these were the incremental backup pieces, multiple files because I'm using parallelism of 4.

Under a /FRA/<SID> I see 3 directories: archivelog, autobackup, and backupset. I assume arhivelog is where Oracle is archiving the redo logs, autobackup is storing the SPFILE and CONTROLFILE as I have enabled controlfile autobackup. However, what is being stored in backupset?

When I do a crosscheck, it lists the files within backupset, but does not list the several files under the root of FRA (b6mbg614_1_1 and so on).

What's the difference between /FRA/<SID>/backupset and the files under root FRA? Do I need both for recovery - and if not, why is Oracle keeping both?