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What is Han unification?
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The Han unification — more properly the Qin-Han unification — of China occurred in 221 BCE. It was a political transformation, from a collection of feudal states to a more centralized system under the emperors of the Qin and then Han dynasties, that had numerous cultural and legal ramifications.
The Cambridge History of China (ISBN 0521243270) describes one such cultural ramification: the redefinition of the geographical extent of China. Pre-unification it had been both defined and understood to be "all under Heaven". Post-unification, however, it became redefined as that which lies "within the seas". Whilst the description "all under Heaven" was still employed, it was used mainly for political expediency rather than because of literal belief in its truth as a geographical description of Imperial China.
In part this because of the First Emperor Qin Shihuang's belief in theories propounded by Tsou Yen, who pictured the world as nine continents, each comprising nine regions, separated from one another by encircling oceans, and who stated that China was the Red Region of the Spiritual Continent.
Planning Chinese Characters: Reaction, Evolution Or Revolution? (ISBN 1402080387) describes, in chapter 1, another cultural ramification: the implementation of a single uniform standard for writing, something that Chinese scholars had been stressing the importance of for the previous two centuries. Prime Minister Li Si oversaw The Burning of All Books, wherein all books, excepting those on medicine or agriculture, written pre-unification in non-standard scripts were destroyed. The Qin government instituted a programme, perpetuated by almost all subsequent dynasties, of rationalization and promulgation of standard character shapes, rectifying variant forms and rejecting non-standard characters, creating and publishing textbooks and dictionaries, and publicizing the standard characters by engraving classic texts written in those characters on stone tablets.
The process by which the Chinese, Japanese, Korean, and Vietnamese character sets were rationalized by the Unicode consortium is sometimes erroneously named Han unification.