On the basis of states of matter, deposition process can be divided into four types: solid, liquid, air, and ion. This division guarantees that no process will be left out from a classification, as every process deals with a feedstock that has a state of matter. What if a process deals with more than one state of matter? For example, extrusion-based deposition process works with polymer that has a solid state. Besides, the process works with ink and liquid metal that have liquid states. The process, therefore, will come under both solid and liquid deposition, resulting in it not having a unique place in the classification. Thus, using the state of the matter first and then using the process type does not lead to a unique place. Why not classify the other way round—process type first and then the state of matter? How to know a process type? To know it, it needs to know what the deposition mechanism for a deposition process is.