Nouns in possessive case, genitive case, and attributive noun-noun constructs can look similar, some can be substituted for one another, can even have very similar meaning, yet they do differ.
In most case, at least as I understand it, genitive case (noun1 of noun2) can often be expressed by "noun2 noun1" construct, in which noun2 plays the attributive role. For instance,
A lover of dogs (a person who admires canines) -->
A dog lover (dog is attributive)
An engine of a car (that hunk of metal under the hood) -->
A car engine (car is attributive)
Proper nouns can be attributive ("Moscow streets" = "streets of Moscow" "Canada geese", although not "geese of Canada").
Possessive case is used to indicate exclusive ownership (of an instance or of a class), authorship, etc.
Newton's laws of physics
StoneB's answer to your question on ELL
As far as your question goes, if we take the two specific examples, we see that
a data region's starting point -- is completely the same as
the starting poitn of a data region
because every data region has the starting point, and that point has only one responsibility - to start the data region, so it's exclusive to data regions in general and to each data region in particular.
You could say
a data region starting point
in which case "data region" becomes attributive, which describes the starting point, but in your situation I think it is less desirable because, first, hard to judge to what the indefinite article applies (a data region? a starting point?), and second, it implies that there can be several "starting points" and one (or maybe several) have the attribute "data region". In reality it's a data region in which you're probably interested foremost.
Let's look at page content. You can say
the content of the page -- that would be the same as
the page content
and everybody knows that you're talking about content whose main (for now anyway) attribute is that it represents a page. It doesn't belong to it, though, according to that phrase. But what if you write
the page's content -- is that so wrong?
No, it's perfectly alright because the content cannot belong to anything but the page (of which it is the content). So those three are interchangeable, I believe.
As to "segment boundaries" and "segment's boundaries" (no, "segments boundaries" is non-grammatical), there is a difference in meaning. The former means "boundaries between segments" whereas the latter mean "boundaries of a single segment" (its own boundaries, not necessarily involving other segments).
Knowing the terms now (Possessive/Genitive Case, Attributive noun) you can find more information on those online, like on Wikipedia on elsewhere.