This feature is only available of the Premium and Enterprise subscriptions.
Overview
The purpose of subforms is to allow an entrant to submit multiple entries within a single upload. Without subfoms, the entrant is required to repeatedly complete all fields in the entry form for every entry they want to submit.
The common use-case for subforms are competitions where a teacher may submit multiple entries on behalf of their students or photo competitions where the same user wishes to enter the contest with multiple individual entries.
How does it work?
The subforms works by grouping fields into 2 types:
Fields that are common to all the entries. Typically this would be contact information such as name, email address, age, address, grade or any other data about the entrant.
Fields that are specific to each entry. This would include fields such as, the title, the media to be uploaded, a description of the entry, etc.
This grouping is demonstrated the Form Designer below. The contact details and Terms and Conditions checkbox are outside the subform and will be applied to all entries. The Upload (media), Title and Description fields will be supplied for each entry.
To the entrant, the subform makes the entry form look like a single submission for multiple entries. However, once submitted, the individual entries are separated, with the common data assigned to all entries. As an Administrator, the entries will appear as unique items in the Moderation queue and Entry table - the bulk nature of their upload will be transparent to you.
Below is a screenshot of how the form may look on the client-side:
When the button Add Another Entry is pressed, multiple entries can be added to the submission form. Each entry will still be accessible via an expandable panel. The example below shows the #1 entry has been entered (an is now in a collapsed state) with #2 entry being entered.
Contents of the subform can be inserted, edited or deleted prior to the submission of the form. The entry panel can be expanded by clicking the 
icon in the top right of the panel. Expanding an existing subform entry to be edited.
The entry can be removed from subform by clicking the 
icon located next to the expand/collapse icon. The screenshot below, shows #1 being expanded.
Adding a Subform to your Upload Form
To configure a subform for your contest, complete the following steps:
Navigate to the Form Designer
Contest > Manage > Settings > FormsPress the Manage button for the Upload form to edit
Press the green Add button in the top right hand side. A dialog menu will appear.
Select the option Add Sub Form.
You can now configure the SubForm, giving it a label (in this case Entry Specific Fields) and a maximum number of entries that can be uploaded with the submission. Note: There is currently a platform maximum of 10. If this is an issue please contact Support.
Press the Save button. You will return to the Form Builder and the form will now have 2 new rows: <Your SubForm Label> - Start and <Your SubForm Label> - End.
Use drag-and-drop to place the desired fields (rows) between the Subform's Start and Stop rows. These fields will now be treated as entry specific field.
Please note there are some limitations:
The core email address field cannot be located in the subform - all entry submissions must belong to the same email address.
While the Category field can be included in the subform, it will only work if all categories use the same default form. If you've configured your categories to use different forms, then the Category field cannot be placed in the subform.
The form saves automatically when each row is moved. Once all the required fields are positioned in the Subform your done!