POLAND AND HUNGARY have now taken another long step into the world economy with their applications to join the International Monetary Fund. They are similar economies in very dissimilar situations, coming to the IMF for quite different reasons. Poland, deeply in debt, its production profoundly disrupted by more than a year of political turbulence, needs immediate help on a substantial scale. For the Poles, the IMF is the most promising prospect for the kind of immediate financial support that can keep commercial lines open to Western Europe through the winter.

For the Hungarians, the need is less urgent and the view is longer. Membership in the IMF will enhance the country's credit and assist its progress toward convertibility of its currency and the increased trade that will follow. Hungary has already gone a long way in opening its economy to the larger European trading system. Half of the country's trade is already with the West, and participation in the IMF implies a further shift in that direction.

These things are happening with at least the acquiescence of the Soviet Union, which has apparently decided that the way to restrain the Polish style of discontent is to permit faster economic growth through wider commerce with the West. But selling to sophisticated markets like Western Europe's requires the seller to meet the markets' standards of quality and style. It's very difficult for a tightly controlled economy to deal with an open market; trade means decentralization. It's been true since the time of the Phoenicians that the pattern of trade means more than money.

When the Soviet Union forced Czechoslovakia to drop out of the formulation of the Marshall Plan in 1947, it was a decisive and profoundly ominous signal of Soviet intentions for postwar Europe. It announced a determination to seal the Russians' clients off from the rest of the world. Just as that policy held immense historical significance, so the gradual relaxation--under way for many years and now accelerating--deserves equal attention. It is also, incidentally, another demonstration of the usefulness of the IMF, an institution that the Reagan administration often seems to regard with a degree of suspicion. Among its other contributions, the IMF helps many kinds of countries, under many kinds of economic regimes, gain access to the flows of world trade--and all that comes with it.