We call an “Embedded” list one that utilizes the Flex aspx forms and connects to other Flex list items – such as Accounts, Contacts, Interactions, etc. An Embedded list item can play different roles within a Flex system depending on the purpose of the list.
For example, an Embedded list might be created for tracking training programs or orders where existing list items such as tasks, interactions, emails and documents need to be attached and where these items will relate to parent items such as Organizations, Individuals or Projects.
An Embedded list will need to have the 4 primary ID fields shown below added to its List Settings.
An Embedded list is created in the same way as the other Custom list types, but it will require the SharePoint Designer step, along with potentially Action Bars, Custom Menus and Quick Actions.
See section on Creating a Custom List for steps on how to do just that.