Can endo and exo products occur when the diene is not cyclic?

I have always thought that the diene needs to be cyclic for there to be *endo* and *exo* products. Perhaps, I formed this impression because most textbook examples that are used to illustrate the concept of the *endo* preference in Diels-Alder reactions involve a cyclic diene. Recently, I came across [this website](https://www.masterorganicchemistry.com/2018/02/09/endo-exo-diels-alder-telling-them-apart/) and if you look at Point 7, Example 1:


[![enter image description here](https://i.stack.imgur.com/7fXLJ.png)](https://i.stack.imgur.com/7fXLJ.png)


Firstly, as pointed out by someone in the comments section on that webpage, the use of the terms "endo" and "exo" are incorrect, as the products are not bridged. Thus, according to IUPAC rules, the terms cannot be applied and instead, "endo-like" and "exo-like" may be used. However, this is not my point of contention.


Perhaps, I simply lack the ability to visualise this but can someone explain to me how the aldehyde substituent would be considered to be nearer to the double bond in the stated "endo" product, compared to the stated "exo" product.


In general, I would also like to ask if there exists an *endo* preference in Diels-Alder reactions where the diene is not cyclic.


