What does "if not necessarily the letter" mean? the letter = spirit?

Context:
As with previous deals involving other American companies, critics argue that Bain’s acquisition of Uniview violates the spirit — if not necessarily the letter — of American sanctions imposed on Beijing after the deadly crackdown on protests in Tiananmen Square. Those rules, written two decades ago, bar American corporations from exporting to China “crime-control” products like those that process fingerprints, make photo identification cards or use night vision technology.
More:
http://www.nytimes.com/2012/03/16/world/asia/bain-capital-tied-to-surveillance-push-in-china.html?_r=1&hp