China is present at the Cancun summit, and the Soviet Union is absent. Moreover, Cancun marks Premier Zhao Ziyang's debut as an international statesman.
He has made a couple of visits to Asian countries since taking over the premiership 14 months ago, but this is his first world summit. For the Chinese, therefore, the 22-nation North-South summit for which Mexico is playing host Oct. 22 to 23 is an important occasion.
As a developing nation with a per capita gross national product of $250 a year, China places itself squarely with the South - that is, with the developing nations of the so-called third world. Mr. Zhao said before he left that China ''shared weal and woe with other developing countries'' and supported their demand for ''global negotiations.''
At Cancun, therefore, China is automatically in a position different from that of the United States, the world's leading economic (as well as military) superpower.
A commentary by the official New China News Agency Oct. 20 dismissed as ''empty talk'' President Reagan's recent speech saying American relations with developing countries ''play a critical role'' in US foreign policy.
But this is mild criticism compared to its attacks on the Soviet Union for not coming to Cancun and not doing more for the developing nations. In this sense, Premier Zhao's presence at Cancun is a plus for American foreign policy.
Many other developing countries concentrate their fire at North-South meetings on the Western industrialized countries, particularly the United States , while ignoring or soft-pedaling criticism of the Soviet Union. Not so China.
A Chinese commentary Oct. 20 pointed out that military aid accounted for two-thirds of the $47 billion in aid the Soviets supplied to 73 developing countries from 1954 to 1978.
''The Soviet Union has thus become the world's second-largest merchant of death next to the United States,'' the commentary says.
''From the early 1960s to the late 1970s,'' the article continues, ''the Soviet Union sold at high prices over $20 billion worth of machinery equipment to the developing countries in the form of economic aid, grabbing staggering superprofits.''
A commentary in the People's Daily Oct. 21 said, ''Moscow's so-called foreign aid is like pulling one piece of hair from nine cows. Having plucked but one strand of hair from its own body, it trumpets it abroad as if the heavens were filled with fluttering blossoms. . . . But once the time comes to discuss the developing countries' demands for global negotiations, like a woman with bound feet, it cannot go forward, fearing lest it have to give up several more strands of hair.''
At Cancun, Premier Zhao will have his first tete-a-tete with President Reagan , plus a series of mini-summits with other leaders, notably Indian Prime Minister Indira Gandhi.