You can Show/Hide the fields to portal users based on condition match that is set and configured in the “Conditional Settings” for the fields. When the portal user selects a Field, which is set as Conditional Field, the Field which is selected in “Visibility Field” will appear in add/edit of any modules. 
Priority: Select the priority of the subject from “High”,” Medium”, and “Low”.
If the customer has selected the “Low” priority, then the Type field will appear while adding the case as per the conditional statements from the Conditional Settings. 
QA Radio Option: The portal users can select the required option. When the portal users select the options that is set for the conditional field, based on that any other field will visible, the portal users will get another field.
I.e. By selecting the “option 1” from the QA Radio Option, the portal users will get the QA Multiselect field.
QA Multiselect: If the portal users select the “Option 1” and “Option 2”, so based on the “And” condition, the Status field will appear after the “Description” field. 
Now, the portal users can select the Status as per requirement. After adding the details, the portal users can save the case details.