Some custom actions may not appear in the Actions list inside the Perform Action step of a workflow in Dynamics 365 because they use parameter types that are not supported. These unsupported types include Picklist (Optionset), Entity, and Entity Collection. Only actions with input as Entity Reference are listed, as it is a supported type. Actions with unsupported types may still be listed if used as an output parameter. For more information, refer to the provided link.
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