Full, incremental or differential backup? Using RMAN my database is backed up once a day using a command similar to this

backup incremental level 1 cumulative
[...]
database
include current controlfile spfile


In the recovery catalog database I can see one of these backup jobs like this 

    DB_KEY DB_NAME  SESSION_KEY COMMAND_ID              START_TIME          END_TIME            INPUT_BYTES OUTPUT_DEVICE_TYP STATUS        INPUT_TYPE    ELAPSED_SECONDS
---------- -------- ----------- ----------------------- ------------------- ------------------- ----------- ----------------- ------------- ------------- ---------------
   xxxxxxx dbname       xxxxxxx 2016-11-12T05:03:21     12.11.2016 05:02:28 12.11.2016 05:03:21   862159354 SBT_TAPE          COMPLETED     DB INCR                    53


In the table rc_backup_piece_details there are five entries with this backup job's session id

SESSION_KEY DB_NAME      DB_KEY     BP_KEY B INCREMENTAL_LEVEL DEVICE_TYPE          START_TIME          COMPLETION_TIME     ELAPSED_SECONDS S      BYTES
----------- -------- ---------- ---------- - ----------------- -------------------- ------------------- ------------------- --------------- - ----------
    xxxxxxx dbname      xxxxxxx    1000001 I                 1 SBT_TAPE             12.11.2016 05:03:14 14.11.2016 05:03:15               1 A     423549
    xxxxxxx dbname      xxxxxxx    1000002 I                 1 SBT_TAPE             12.11.2016 05:03:05 14.11.2016 05:03:11               6 A  862156486
    xxxxxxx dbname      xxxxxxx    1000003 D                   SBT_TAPE             12.11.2016 05:03:16 14.11.2016 05:03:17               1 A   45324586
    xxxxxxx dbname      xxxxxxx    1000004 I                 1 SBT_TAPE             12.11.2016 05:02:30 14.11.2016 05:03:01              31 A  256422458
    xxxxxxx dbname      xxxxxxx    1000005 I                 1 SBT_TAPE             12.11.2016 05:03:13 14.11.2016 05:03:14               1 A   45624595


As you can see, there is one backup piece wich D as backup type. In the Oracle documentation I found the following:


  The type of backup. Possible values are D for data file or control file backups, I for incremental backups, and L for archived log file backups


Does this mean, that among these backup pieces there is one incremental backup of a data file or control file? 

If this is the case, then I am wondering, why this only happens with this particular database. All other databases are backed up with include current controlfile spfile too.

The difference between this and other databases is that most other databases are backed up incremental level=0. So might there be the difference?



Edit: So, if backup type D is not incremental backup, then why is this backup piece contained in a backob job with input_type = 'DB INCR'? AND could  what part of the rman command triggers this datafile backup and if it is include current controlfile spfile then why is there a backup piece with backup type D for any other database?
Does this mean, that among these backup pieces there is one incremental backup of a data file or control file?


No, the Backup type is D means it's backup of data files or control file but it's not incremental its full backup(denoted by NULL for INCREMENTAL_LEVEL).

Other backups are differential incremental backup of the backup type D which is base for these incremental backups.


  The difference between this and other databases is that most other databases are backed up incremental level=0. So might there be the difference?


Incremental Level 0 and Incremental Level 1 Cumulative is different types of backup so there is difference. 


  Incremental backups can be either level 0 or level 1. A level 0 incremental backup, which is the base for subsequent incremental backups, copies all blocks containing data, backing the datafile up into a backup set just as a full backup would. The only difference between a level 0 incremental backup and a full backup is that a full backup is never included in an incremental strategy.


For more: RMAN Incremental Backups