The Zohar explains the correction of the lines in the vessels of Zeir Anpin and Nukva (ZON).
After clarifying the order of the propagation of the Mokhin from the perspective of the lights -according to the three corrections Holam, Shuruk, Hirik  - it proceeds to clarify the order of the propagation of the Mokhin from the perspective of the vessels, which are called the three lines: right, left, and the middle line that decides between them.

There are many designations for the ten Sefirot, according to their relationships.
From the perspective of the clothing of the lights in the vessels, the ten Sefirot are called Hokhmah, Binah, Tiferet, Malkhut (HB TM), or Hesed, Gevurah, Tiferet, Netzah, Hod, Yesod–Malkhut (HGT NHYM).
And from the perspective of the vessels, the ten Sefirot are called the twenty-two letters.

According to this, they should have been five letters (HB TM) or seven letters (HGT NHYM).
Why then are they twenty-two?

Because the letters extend from the seven Sefirot HGT NHYM of Aba ve Ima (AVI).
And since there are three discernments in their Mokhin—Holam, Shuruk, Hirik—three types of propagation of the seven vessels extend from them:

1. Seven vessels extending from the point of Holam, corresponding to the right line.

2. Seven vessels extending from the point of Shuruk, corresponding to the left line.

3. Seven vessels extending from the point of Hirik, corresponding to the middle line.

Together these make twenty-one vessels.

However, in the point of Hirik, there is an additional discernment beyond the seven Sefirot HGT NHYM of AVI: this is the discernment of the Massakh (screen) of ZON.
This additional discernment is also regarded as a vessel.

Therefore, there are twenty-two vessels, which are called the twenty-two letters.

Thus, the twenty-two letters are essentially one matter with the seven lower Sefirot (ZAT).

But from the perspective of the vessels themselves, the Sefirot are called “letters,” numbering twenty-two, due to the correction of the three lines in them, which allows their propagation and clothing.

Excerpt from “Zohar for All,” Bereshit, Article “Let There Be Light.”