The idea behind RETs is to quantify complex data relationships maintained in a single FTR.
Record element types are one of the most difficult concepts in function point analysis. Most record element types are dependent on a parent - child relationship. The child information is a subset of the parent information. In a parent child relationship there is a one to many relationship.
Imagine a customer file that contains Name, Address, so on and so forth. In addition all the credit cards and credit card numbers of the customer are contained in the file. This would be an example of 2 record types. There would be multiple occurrences of credit cards and numbers for each customer. The credit card and numbers are meaningless when not linked to the customer.