one block is made of:
one record 10 (customer)
1 to N record 20 per record 10 (accounts)
1 to N record 30 per record 20 (transactions)

record type 10
fields:
record type on 10 characters
customer id on 10 characters
customer lastname on 20 characteres
customer firstname on 30 characters

record type 20
record type on 10 characters
customer id on 10 characters
account id on 10 characters

record type 30
record type on 10 characters
customer id on 10 characters
label on 10 characters
value on 10 characters