Custom fields allow you to add an attribute to metadata items that aren't included in the standard. For example, a pluralised name for Object Class is a common custom field not available in the standard.
1. Go to "Administrator tools" from your Dashboard side panel.
2. Select

3. Select whether you want to add a custom field to "All" metadata item types:
Or, a specific metadata item type:
In this example, we are going to add a custom field for a "plural" to an Object Class.
4. Select

5. Then,

6. Add a Name, the Type of data stored in the field, the System name used for API's, and Help text (if required to describe the field.
7. You can also add in the "Visibility" - who is this custom field going to be visible to
And, the "State" - this can be used to be disable the field, to prevent people from using it.