Yocto wic Creates Unexpected Small Partition - linux

Question

I am using Yocto and it's wic tool to build my embedded Linux image.
The wic configuration file looks like this:
part /boot --source bootimg-partition --ondisk mmcblk --fstype=msdos --label boot --align 1024 --fixed-size 64
part / --source rootfs --ondisk mmcblk --fstype=ext4 --label root_a --fixed-size 256 --active
part / --source rootfs --ondisk mmcblk --fstype=ext4 --label root_b --fixed-size 256
part /permanent-storage --ondisk mmcblk --fstype=ext4 --label permanent-storage --fixed-size 300
part swap --ondisk mmcblk --size 64 --label swap --fstype=swap
I burn the resulting image to my SD Card and boot successfully, and there is an unexpected small ( 1K ) partition:
root#eval:/dev# ls -lrt /dev/mmcblk0*
brw-rw---- 1 root disk 179, 0 Feb 27 21:55 /dev/mmcblk0
brw-rw---- 1 root disk 179, 4 Feb 27 21:55 /dev/mmcblk0p4
brw-rw---- 1 root disk 179, 2 Feb 27 21:55 /dev/mmcblk0p2
brw-rw---- 1 root disk 179, 3 Feb 27 21:55 /dev/mmcblk0p3
brw-rw---- 1 root disk 179, 5 Feb 27 21:55 /dev/mmcblk0p5
brw-rw---- 1 root disk 179, 1 Feb 27 21:55 /dev/mmcblk0p1
brw-rw---- 1 root disk 179, 6 Feb 27 21:55 /dev/mmcblk0p6
root#eval:/dev# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
mmcblk0 179:0 0 59.6G 0 disk
|-mmcblk0p1 179:1 0 64M 0 part
|-mmcblk0p2 179:2 0 256M 0 part /
|-mmcblk0p3 179:3 0 256M 0 part
|-mmcblk0p4 179:4 0 1K 0 part
|-mmcblk0p5 179:5 0 300M 0 part
`-mmcblk0p6 179:6 0 64M 0 part
Why is wic creating this partition and how can I get rid of it with my wic file? Thanks.

Answer 1

The mmcblk0p4 (1K) partition is an extended partition. When using a master boot record (MBR) to partition storage into more than 4 partitions one must use 3 primary partitions and 1 extended partition. This is because there is a maximum of 4 primary partitions. The extended partition may hold multiple logical partitions.
mmcblk0 <- Entire Storage
|--mmcblk0p1 <- Primary Partition 1
|--mmcblk0p2 <- Primary Partition 2
|--mmcblk0p3 <- Primary Partition 3
|--mmcblk0p4 <- Extended Partition
|--mmcblk0p5 <- Logical Partition 1
|--mmcblk0p6 <- Logical Partition 2
This is not Yocto specific. I use Buildroot and have a similar layout. The commonality is the disk partition method not the Linux distribution.
Wikipedia: Disk Partitioning
Serverfault: Primary vs extended partitions on Linux