The requiestor is making a point that there is no "relationship" to which attach such a source in FTM - no structure for documenting relationships. Attaching a citation to a birth fact doesn't document the relationship, within the program.
What I use is Custom Facts and it works well for me. I have the following Custom Facts:
Link to children
Link to parents
Link to maiden name
Link to spouse
Link to siblings
Link to others
In the case of a birth certificate that names the parents, I use:
In child's record
Link to parents: John's birth cert names parents: Roy and Dale Rogers
In each of parents' records:
Link to children: John Roger's birth cert shows parents: Roy and Dale Rogers.
I will then attach the citation that you mention to each of these custom facts. (as well as to name facts and birth fact.)
Of course, one is not finished. The Big Three genealogy assertions are:
How does one know x is child of y and z and/or vice-versa
How does one know spouse of x is spouse of 1
How do we know x in one record in a place at one point in time is the same x in another record at another place at another point in time.
But, we are well on the way to documenting the relationships of that person.