Please answer a question about the following article about Korean War:

On 27 September, MacArthur received the top secret National Security Council Memorandum 81/1 from Truman reminding him that operations north of the 38th parallel were authorized only if "at the time of such operation there was no entry into North Korea by major Soviet or Chinese Communist forces, no announcements of intended entry, nor a threat to counter our operations militarily..." On 29 September MacArthur restored the government of the Republic of Korea under Syngman Rhee. On 30 September, Defense Secretary George Marshall sent an eyes-only message to MacArthur: "We want you to feel unhampered tactically and strategically to proceed north of the 38th parallel." During October, the ROK police executed people who were suspected to be sympathetic to North Korea, and similar massacres were carried out until early 1951.

What memorandum stated the circumstances for engaging in conflict above the 38th parallel?
National Security Council Memorandum 81/1