Currently, you can link a contact directly to a Custom Data Object/Data Card (CDO), but you cannot link a CDO to another CDO. By allowing this, you can drill down two levels and create even more accurate segments.
Our client Joe owns three accounts, and each are a different type of account. Within each of his accounts, he has transactions in various currencies, including the Euro.
Our client Sue owns three accounts, and each are a different type of account. Within each of her accounts, she has transactions but only in USD.
This is the relationship of the objects in our CRM. Contact owns Account, and Accounts owns Transactions. Contact can own many accounts. Account can own many transactions. BUT, a Contact cannot never own Transactions directly.
Currently, I can successfully create a segment using "Has Linked Contact in Custom Object" to show all contacts associated with Accounts where Type = Personal. However, it is not possible to link a contact directly to a transaction.
If I could link the Transactions (CDO) to the Accounts (CDO) by their own shared unique identifier (Account ID), then I could tell Eloqua "Show me a list of contacts associated with personal accounts, that have transactions in Euros." In that case, of my two example contacts, only Joe would qualify.