ARP1331C results from a Five-Year Review and update of this specification with the addition of alloys in the Scope (1), addition of Intended Use (1.3), added definitions of Hypermilling and Transition Zone (2.2), added application to additively manufactured metal parts (3.1.1), added application for removal of residual metal (3.1.6), added NOTE regarding residual stress (3.2.7.1), added two Advantages (3.2.13, 3.2.14), added three Limitations (3.3.9, 3.3.10, 3.3.11), added common milling techniques to Depth of Removal Tolerance (4.1.2.2.1), added suggestions for reducing bath heating for light weight applications (4.1.6.1), added Transition Zone limits (4.1.12), expanded materials for Template Materials (4.2.1), added an example for Undercut Rate (4.2.3), expanded requirements for Cleaning (4.3.1.1), added “copper” to list of materials that need specific cleaning steps performed (4.3.1.2), clarified rinsing instructions (4.3.1.3), added two limitations to Masking (4.3.2.5, 4.3.2.6), added two limitations to Scribing (4.3.3.6, 4.3.3.7), revised the etch/metal rate determination (4.3.4.10), added direct dimensional verification (4.3.4.13), added note regarding surface roughness (5.3), added ARP4992 to the notes for control of processing solutions, and added Figure 4 showing an example of transition zone.