C = union (A, B) returns the unique common values of the categorical arrays A and B. Either A or B input arguments may be a character vector, a string array, or a cell array of character vectors, which is promoted to a categorical array prior to set exclusive-or. If both A and B are row vectors, then C is also a row vector, otherwise union returns a column vector.

If categorical arrays A and B are ordinal, they must have the same set and ordering of categories, which is transfered to C. If neither are ordinal, the category names of each pair of elements are compared (they do not need to have the same set of categories) in which case the categories in C are the sorted union of the categories in A and B.

C = union (A, B, 'rows' returns the unique common rows of the categorical matrices A and B, which must have the same number of columns. By default, the rows in categorical matrix C are in sorted order.

[C, ixA, ixB] = union (…) also returns index vectors ixA and ixB such that C = A(ixA) and C = B(ixB), unless the 'rows' optional argument is given, in which case C = A(ixA,:) and C = B(ixB,:).

… = union (…, order) also specifies the order of the returned unique values. order may be either 'sorted', which is the default behavior, or 'stable', in which case the unique values are returned in order of appearance.

Source Code: categorical