What is ETB0 in debugfs output

I'm doing some research on ext4 filesystem low-level details. Specifically on how block-level data is stored and addressed by inodes.

Getting the information on block sequence addressed by some inode this way:

$ sudo debugfs /dev/nvme0n1p5
$ stat <inode_num>

It looks like directories and regular files has some difference on how blocks are stored:

Directories ( eg /var/log in my case) has single block entry:

Inode: 6029320   Type: directory    Mode:  0775   Flags: 0x80000
Generation: 2262719563    Version: 0x00000000:000021d1
User:     0   Group:   108   Size: 4096
File ACL: 0    Directory ACL: 0
Links: 18   Blockcount: 8
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x5dcc233c:99352690 -- Wed Nov 13 17:37:32 2019
 atime: 0x5dcc23c9:7adfbb20 -- Wed Nov 13 17:39:53 2019
 mtime: 0x5dcc233c:99352690 -- Wed Nov 13 17:37:32 2019
crtime: 0x5b2ac0de:4e871810 -- Thu Jun 21 00:02:22 2018
Size of extra inode fields: 32
EXTENTS:
(0):24125478

Regular file (/var/log/syslog) has multiple entries:

Inode: 6029451   Type: regular    Mode:  0640   Flags: 0x80000
Generation: 3263261578    Version: 0x00000000:00000001
User:   104   Group:     4   Size: 32009262
File ACL: 0    Directory ACL: 0
Links: 1   Blockcount: 62528
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x5dcc2482:6eb68abc -- Wed Nov 13 17:42:58 2019
 atime: 0x5dcc1ce5:1c3a8e0c -- Wed Nov 13 17:10:29 2019
 mtime: 0x5dcc2482:6eb68abc -- Wed Nov 13 17:42:58 2019
crtime: 0x5dc28c65:36d636f4 -- Wed Nov  6 11:03:33 2019
Size of extra inode fields: 32
EXTENTS:
(ETB0):24176616, (0):876526, (1-2):894872-894873, (3):894879, (4):894446, (5):893947, (6-7):102005-102006, (8-9):620027-620028, (10):602618, (11):620029, (12-13):626677-626678, (14-15):102036-102037, (16-31):12846176-12846191, (32-63):14649408-14649439, (64-255):13046208-13046399, (256-511):13244672-13244927, (512-1023):13309440-13309951, (1024-2047):1545216-1546239, (2048-4095):14821376-14823423, (4096-6143):9828352-9830399, (6144-7814):15056896-15058566

I could suppose the directory is storing relatively small amount of data (filenames and inodes entries only) so it may benefit a single non-divided block space which is hardly possible with regular file with a lot of data.

What I would like to know is how blocks are addressed in the case of regular file. Isn't it should start from 0 index? Why block with ETB0 index appears first in the listing? And what is ETB0 meaning in regular file and why it absent in the directory?