Your customers can create different kinds of custom fields for both Styles and Items.
VIEW CUSTOM FIELDS
When they create the custom field, they can define any field to be viable for you.
They can also allow you to fill in data. If that is the case, you will be able to see the edit icon (see image below).
EDIT CUSTOM FIELDS
The Custom Field(s) you are able to edit will become editable. Simply:
1) Type in the needed value(s)
2) Click on the save button