Use this command to select all items in the selected object's or objects' layer that have the same style(s), as defined in AutoCAD Architecture (formerly known as ADT, or Architectural Desktop). For example, if you've imported or linked to a DWG file that contains walls in several different styles, such as CMU-8, Concrete-8, and Stud-4, you could select all CMU-8 wall segments in the same layer by selecting one and then invoking Select Similar.
If you start by selecting multiple objects with different styles, Select Similar will select all objects with those styles.