Custom fields allow the administrator to include additional data about domain accounts. The fields will be displayed when editing the domain. They can be used for just about any type of data. For example, it may be useful to have additional contact information attached to the domain account, or a notes field used to explain why a domain is disabled or has some additional feature or limit beyond other domains created with the same template. If your are developing tools to interact with Virtualmin, these fields may be useful for other purposes.