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You can enable related lists on custom input forms where the form type is View or Input & View. When a user views a document using a custom input form, any related lists that have been enabled on the form are displayed in a panel across the bottom of the page. For example:
The number indicates the number of items in that related list. Open and close a related list by clicking its name. When open, the panel expands to display the contents of that related list. Any available standard Salesforce buttons (such as New) are displayed across the top of the panel. Links to standard actions are displayed in the Actions column. Clicking on these buttons and actions may load standard Visualforce pages but will return you to the custom input form. You can also use the 
icons to expand and collapse the panel.
For information about the standard Salesforce related lists that are available, see Standard Salesforce Related Lists on Custom Input Forms. For information about enabling related lists and creating custom related lists, see Managing Related Lists on Custom Input Forms.