Can a custom multi-select picklist be used as a controlling field?

A custom multi-select picklist cannot be used as a controlling field because controlling fields in Salesforce must be single-select picklists. This is due to the nature of controlling fields, which define what values are available in dependent fields based on the selection made.

When using a multi-select picklist, the user can select multiple values, which complicates the determination of which value should control the display of dependent fields. The selection of multiple values does not provide a clear single choice from which to derive the dependent values.

In Salesforce configuration, controlling fields are expected to have a clear singular selection, allowing for a straightforward mapping to dependent picklist values. For a picklist to effectively control another field, it must present a single value that can logically determine what options are available in the dependent field. Hence, a multi-select picklist does not fulfill this criterion.