Full recourse loans attempt to assert an ownership interest in persons, as opposed to (or in addition to) objects
Here is the Magisterium on the specific question (Pope Callistus III (1455-1458), Usury and Contract for Rent), describing a morally licit contract (full citation here):
But the [lender], on the other hand, even though the said goods, houses, lands, fields, possessions, and inheritances might by the passage of time be reduced to utter destruction and desolation, would not be empowered to recover even in respect of the price paid.
Even if the value of the collateral goes to zero, it was specified in the contract that that specific collateral is all the non recourse lender is entitled to recover
That is, a licit income-producing rental contract (which might or might not be labeled a ‘loan’ in modern language) is non recourse.
37) I see that the Magisterium and Aquinas have actually been clear that lack of explicit recourse to real assets is central to usury: that full-recourse lending for profit is what is defined as the moral problem. But why is that the case?
Usury involves treating people (subjects) as things (objects), because it involves purchasing “Bob owes me principal and interest” as opposed to purchasing shares in that project there or that bundle of assets there, distinct from particular persons. The most extreme form of treating persons as property is chattel slavery. (Some authors beg to differ, seeing usury as worse, and the argument has some merit). Usury is in the same moral genus as slavery.
Furthermore, “Bob owes me principal and interest” is not a thing which actually exists. Charging rent for literally nothing, no thing, is intrinsically unjust.
38) But you’ve said that intangible or only partly tangible things like patents and operating businesses can be ‘objects’, and thus can be property. So how do I tell the difference between what can be ontologically real property and what can’t?
Ontologically real property consists of objects. (Property in general refers to a relation between owners, subjects, and objects; but what we ordinarily call ‘property’ as a noun are the objects in this relation). Objects can be alienated from persons and possessed or controlled by different people at different times.
Modern economics is very anti-realist: it is under the delusion that economic value is purely subjective, that is, purely a function of human preferences whatever they happen to be. But economic value is not purely subjective: it has an ineliminable objectivity. (Modernity in general is characterized by anti-realist materialism).
Objects, very generally speaking, are things which have an existence that is independent of particular persons (subjects). The contrary of object is subject, so objects are things that exist in their own right independent of persons: things which are not persons and which are capable of being exchanged independent of particular persons.
Non recourse loans represent ownership claims in objects: the specified assets to which the lender has recourse (under the terms of the contract) to recover principal and interest.
That is what the lender agreed to, by the definition of a non recourse contract
Notice that when Bob dies, the ‘value’ of his full recourse debt (qua full recourse) dies with him. The value of any non recourse debt contracts does not die with any particular person, precisely because that value is tied up in the specific objects not in a particular subject (person). Everything that a non recourse lender is entitled to under the contract can always by definition be recovered (absent theft, fraud, vandalism, or other criminal acts) from objective reality.
Full recourse lenders frequently fail to fully recover their contractual entitlements precisely because those entitlements are to ‘things’ which are not real.