When you add a custom table you may need to attach multiple contacts, companies, groups and/or opportunities. Using any one or a combination of these four controls you can attach the custom record to each.
To add one of these controls open the form in the designer and ollow these steps:
Click on the "Parent-Child icon on the top toolbar
From the TOOLBOX list on the left side select your desired entity
Drag the control onto the form
Use the PROPERTIES panel set your desired options.