ISO 10646 is a four byte code table that basically assigns sections of the code table to individual alphabets. Characters that occur in more than one alphabet are stored in more than one section - this is especially true for the ideographic alphabets.
To reduce the storage demands for ISO 10646 documents, one- or two-byte subsets of the four-byte code table may be used in a document; control characters are needed to indicate a change of alphabet, e.g. to the four-byte code table.
ISO 10646 has been formally approved in 1992.