Chain length is the number of carbon atoms in a hydrocarbon molecule. A short carbon chain and a long one can have different physical properties and uses, so chain length helps describe which products a chemical process has made.
In Fischer–Tropsch synthesis, syngas is converted into hydrocarbons with a range of possible chain lengths. Controlling that range matters when a process needs to favor particular products rather than produce a broad mixture. The term is also useful in chemistry and materials science when comparing molecules and assessing product selectivity.