The process of populating an external REI form through Rex makes use of a data mapping. This is written manually or in a way that generically targets the fields on each individual form.
Due to the large number of forms which exist on REI (~900 across 6 states) we can't easily map every single form individually. Instead, the strategy we opt for makes use of heuristics where fields provided through REI are mapped from Rex by name.
Using the strategy mentioned above, we can also determine when, what type and how many contacts are required for the form. For example, if we see a collection of fields named "Tenant1_Phone, Tenant1_Fax, Tenant1_name etc." we can recognize that we need to provide tenant contacts. In turn, if we see fields named "Principal, Principal2, Agent etc." we know that Rex agents are required instead of contacts.
Due to the strategy used above, it's possible that form fields may appear as missing or incorrectly mapped. If you're at all convinced this has occurred in any case, please contact support and we will attempt to fix the mapping.