What is "BIOS grub spacer"?

While installing Ubuntu Server in Virtualbox I came across this, and I wanted to know what it is because I am unable to find anything on Google:

enter image description here

Solution 1:

A partition boundary introduced with UEFI. It aligns the partitions.

You did not find this one?

#: ../subiquity/common/filesystem/labels.py:81
msgid "BIOS grub spacer"
msgstr ""

It is partition type 81.