Union calculates the geometric union of any number of feature classes and feature layers. All input feature classes or feature layers must be polygons. The output feature class will contain polygons representing the geometric union of all the inputs as well as all the fields from all the input feature classes.

The Union tool combines input data layers into a single composite layer, preserving the boundaries and attributes from all input features. When features overlap, it creates separate features for them. For non-overlapping parts, these also get passed into the output as well.

union [analysis/geoprocessing] The overlay of two or more spatial polygon datasets that preserves the features that fall within the spatial extent of either input dataset. In a union, all features from both datasets are retained and extracted into a new polygon dataset.

Union combines selected features into one new feature. Although the features may be from different layers, the layers must be of the same geometry type—either line or polygon. Union maintains the original features and their attributes—that is, the selected features are not deleted or edited during the Union operation.