Ionization correction factors (ICFs) are ratios that relate observable ionization measurements to abundances of the element ionized, e.g., in interstellar medium regions such as HII regions. This is the general goal and various strategies are used. They are specific to characteristics of the cloud/object under analysis, including the temperature regime, and the type of cloud/object.
For example, there could be an ICF to relate a particular element's total abundance with its singly ionized abundance to a similar ratio of hydrogen. To handle various situations, the ICFs are derived by functions of other observables, e.g., spectral lines revealing neutral and ionized abundances of additional elements (such as oxygen and sulfur), which incorporate temperature sensitivities. Thus the derived abundance estimate is, ultimately, a function of a number of spectral lines of a number of elements.
The functions to generate ICFs are developed from theory and modeling of photoionization along with observations where some of the "target" abundances can be determined by other means.