Photo chemical etching, chemical milling and electro chemical etching are cousins. In all cases, an etchant is used to remove metal. But, the intent of each process is different.
Photo etching is a fabrication process that yields either finished parts or flat blanks that require additional steps to complete.  Photo etching is not a special process as defined by NADCAP. Photo etching does not in any way alter the physical, mechanical or chemical properties of the alloys we process.
Chemical milling is used to selectively remove metal either from a prefabricated part or to re-gauge a sheet of metal. It is most often used for aerospace components, such as engine nacelles and some airframe structures, to remove non-structural metal in order to reduce the weight of the part. Chemical milling to reduce the thickness of metal sheets is an alternative to grinding. Chemical milling, depending on the application, may be subject to the NADCAP 7108/2 checklist.
Electro chemical etching is a process for marking parts with part numbers, serial numbers, logos, CAGE codes, lot codes and the like.  Electro chemical etching can be applied to many alloys and to curved surfaces. Although specifications for electro chemical etching still appear on many legacy drawings, laser marking has become a popular alternative.
Links to providers of chemical milling, electrochemical etching and part marking can be found here.
A concise list of NADCAP special processes can be found at this link.
