Custom Data Fields are used to store some additional data associated with the step.

Open Step Settings and expand "Custom Data Fields" section.

Click on "add" link to open "Create New Custom Data Field for %step_name% Step" popup. Open picklist "Type" and select data type which field should represent:

Custom Data Field type can be:

• Text - short sting
• Long Text - some long text data
• Decimal - a number
• Date - a date without time
• Date Time - a date with time
• Checkbox
• Picklist - a dropdown list with a set of selectable options and ability to select a default value
• Tag - a field that has only name 

All field types have the mandatory property "Name", that is used to identify the field.

Fields of any type expect "Tag" have the "Default Value" property that is not required.

Field of Picklist type has an "Options" field where options should be listed (separated with semicolon). After "Options" is populated, user become able to select one of them as default value, if needed.

Field of Tag type has no other properties - it's name is used as it's value.

Field of any type expect "Tag" also have properties "Read Only" and "Required", which names describe them enough.

Save configured custom field.

All custom fields will be shown in step properties. Configured fields can be deleted (red cross icon), added to library (paper icon), or edited by clicking on name of proper field:

 Custom Data fields will be shown in the "Data" tab of the proper step on Progress Component:

Custom Data field values can be changed - click "Edit" icon. The field value will be shown in Edit mode. Make changes and click "Save" icon:

Custom Data fields can be used like a step property in different situations, e.g., step actions. They will be listed as fields of "$Properties" Context Object  in Merge Field Picker: