Mark of the Xenos, got it, read it, gonna use it.

The Possessed though have the Daemonic Trait but no number after it. and their stats just reflect the x2 Unnatural Toughness Bonus that Marines get. Should I assume that the Daemonic trait gives them effectively TB 3 (vs norml weapons)?

Now that I look at it, some Daemons have the same issue, while others indicate a TB in paranthesis after the Daemonic trait.

While I believe the daemonic toughness multiplier should be identified in the same manner as unnatural toughness (x2, unusual circumstances the exception of course) in this case it seems the daemonic trait does not confer any additional resiliance.

Rather, it simply means possessed space marines are daemonic and are succeptable to attacks that affect daemons. So if you had a psychic power or weapon that only affected daemons, or does additional damage to daemons, possessed marines would be affected.

Daemonic normally gives Unnatural Toughness (x2) or increases Unnatural Toughness by a step. So those marines should be the latter as well as being vulnerable to anti-daemonic weaponry.

Daemonic (from the book pg. 130)

"Daemonic creatures...

... Creatures with this trait multiply their Toughness Bonus aganst all damage by the number indicated in parentheses after the trait in the same manner as an Unnatural Characteristic ...

... Deamonic creatures are also immune to poison and disease." [sic]

So for possessed marines you add a multiplier to their unnatural toughness as indicated in parentheses after the daemonic trait in its entry. The Possessed Space Marine does not have a multiplier in parentheses (or anything in parentheses) following its Daemonic trait. So you would add 0 to x2 for a total toughness bonus of x2. A Possessed Space Marine is still immune to poison and disease (as it has the daemonic trait).

The additional toughness isn't automatic, you need a multiplier (usually written as "TB 8" to confuse us) in parentheses. A Possessed Space Marine only has a x2 multiplier to its toughness bonus.

That sounded odd to me so I looked at the description in both the Rogue Trader and Deathwatch core books and found that they differ. Fair enough, different systems. But before Deathwatch daemonic worked differently and I wonder if the writer of the Possessed Marine entry forgot that.

At this point I'm assuming its intentional, some Daemons will have the numbers in paranthesis and an equali increase in the over all Unnatural Toguhness and some won't. The possessed space marines will have the poison and other such immunities but no increas in damage resistance, yet at the same time, I suppose they won't lose their resilience against Force Weapons and blessed weapons and such, like some other Daemons that have Daemonic with some numbers and a lower (or none) Unnatural TB. Those creatures will be weaker against Force weapons.

Or am I interpreting that wrong? Basically if there's a number after the Daemonic trait, that number is reduced/negated vs force weapons and holy weapons?

Yes you got it right. Possessed marines have all the immunities of daemonhood but since their toughness is not increased by their daemonic status they still use their full toughness bonus when struck by force weapons and others which ignore daemonic toughness iprovements. So the possessed has 8 TB against a bolter, 8 TB against a force weapon, but only 4 TB against a Felling(1) weapon.