Part 16 · Chapter 96

This is also the secret of the reason for the prohibition of going out beyond the boundary on one's feet, even not by carrying a burden, but the person himself with his feet. And it is so that he not bring out his feet — which are the four last [Sefirot] of Yetzira, the world of the male — to the place of the Klipot, which stand outside the Shabbat boundary, as above. For fourteen Sefirot is their place, and all the rest of the upper place is called the Shabbat boundary, as explained well above. Understand this.