The 13th edition replaced the (Allowable Stress Design) and the 3rd Edition LRFD (Load and Resistance Factor Design) manuals. This "unified" approach allows engineers to use either method side-by-side:
The manual serves as a wrapper for several critical industry standards printed in Part 16: Aisc Steel Construction Manual 13Th 2006.pdf
The Specification for Structural Steel Buildings, which is the heart of the manual. The 13th edition replaced the (Allowable Stress Design)