Ah, perhaps there's a bit of confusion over the difference between the term "form and subform" and the term "subdatasheet".
Also, Referential Integrity is enforced at the table level. The interface (i.e. forms and subforms) can be designed to facilitate it, but they do not enforce it because a user can, at least in theory, bypass your interface.
So, a form is an interface object through which users interact with data in tables. Forms can be bound to the table or to a query based on a table. In some situations, of course, you can embed one form inside another, the "subform". Typically, the main form is this particular interface design is bound to the table on the table on the "one" side of a one-to-many relationship; the sub form is bound to table on the "many" side of that one-to-many relationship. The use of the Parent Child Linking field property in these interface designs facilitates the insertion of the appropriate Foreign Key in the many side table. However, that can not replace the actual table-level enforcement of the relationship. It merely assists you in managing the values.
On the other hand, Access -- in keeping with its penchant for blurring the line between tables and interface elements -- has what are called "subdatasheets" and that's what Colin was talking about.
Here's a screenshot, just to be clear. I normally remove these things immediately upon taking on a new relational database application for the reasons Colin stated. Note that these things are identifiable by the little +/- icon, as highlighted here. What that means is that the ENTIRE table on the many side is dragged into the current recordset along with its parent, an unnecessary burden on resources on your computer. Plus, to me it's much more confusing.
View attachment 99243
Like a lot of things Microsoft has done over the years, it was intended to make it "easy" for inexperienced people to use Access. They munged two different kinds of things together--the "interface" appearance of the subdatasheet and the tables. If you don't find the overhead problematic, they are not so bad, I suppose. But they are ultimately useless, IMO.