Format %s don't give the backupset number

Hello,

I'm configuring scripts for doing backup with rman.

I've configured the format of the files as '%d_backuppiece_%s_%p_%t'

According to my understanding,%s should be replaced by the backupset number.

After some tests, this is not the case.

What am I doing wrong?

I've tried with Oracle Standard Edition 11.2.0.4 and 12.2.0.1.180116

RMAN> CONFIGURE DEFAULT DEVICE TYPE TO DISK;

RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '%d_backuppiece_%s_%p_%t';

RMAN> BACKUP as backupset archivelog all;

Starting backup at 2019-07-04 14:59:09

current log archived

allocated channel: ORA_DISK_1

channel ORA_DISK_1: SID=31 device type=DISK