I'm reading through the document, "The Origins and Evolution of the Palestine Problem, Part II" by the United Nations. Under the provisions of the Palestine Resolution, they note that Palestine was partitioned into eight sections:
The territory of Palestine was divided into eight parts. Three were allotted to the Jewish state, three to the Arab state. The seventh, Jaffa, was to form an Arab enclave in Jewish territory (Annex I).
The eight part was to be Jerusalem as a corpus separatum under a special international regime... (p. 36)
However, the document makes no mention of why this seventh partition was made.
The annex refers to a partition diagram of the region of Palestine. I've copied the relevant portion of the general map here:
In this diagram, orange regions are designated Arab, blue regions are designated for a Jewish state, and the green line represents the UNSCOP's proposed boundary.
What was the motivation behind this seventh partition?