You can manage the Field values and allow the portal users to select the required fields only when they are creating/editing any record in the customer portal.
Advance Field Settings to set default values
You can configure the Advance Field for the Picklist, Status, State, Boolean, Multi-select picklist, etc.. 
If you have inserted the Picklist field in the Form Builder, you can select the default value to display in the picklist drop-down option from the customer portal. 
By editing that Field, you will find the Default Values drop-down option under the ‘Field Configuration’.
Select the default values that you want to show in the customer portal. 
After saving the Field configuration, you can preview the Form to check how portal users will get that option.
Advance Field Management
You can set the condition to the hide/show the fields or multi selection of the Fields. You can set the conditions for the different values to hide or show the related values.
By editing that Field, you will find the Advance Field Management option under the ‘Field Configuration’.
You need to configure Field Management as below: 
Default Field Visibility: It is for the field visible or not. You need to select the ‘default visibility’ for the field values.
Conditional Field: The field on which the current field is show or hide. You can define the hide/show for the selected field as per the ‘conditional field’. Based on the Conditional Field, the default field visibility will change
Multiselect Matching Category: You can select the condition to match from here. There are 3 types of condition provided: 
– Any: If you want to set the condition to match any of the values, select Any.
– All: If all the field of dropdown is selected (only for multi select type), select All.
– And: If all field of Conditional Field is selected within the options of Condition Field Options (only for multi select type), select And.
Conditional Field Option: You need to select the options to be selected for show/hide the field. Select the values on which the conditions should match.
After configuring the Field Management, click on the Save button and publish the form to apply advanced field conditions to the customer portal. You can Preview the Advance Field that configured.