Under the topic "Weakness" in the Rules Reference (p. 21), the second bullet reads:

• When an investigator draws a weakness with a player cardtype (for example, an asset, an event, or a skill weakness) , resolve any Revelation effects on the card, and add it to that investigator's hand. The card may then be used as any other player card of its type.

The term "player cardtype" is confusing me because an Investigator Deck contains two types of weaknesses: a signature weakness and a basic weakness, which is either a Treachery or an Enemy. With a Treachery basic weakness (such as Amnesia), we resolve the Revelation and then put the weakness card in the discard pile; or in cases such as Haunted or Psychosis, the weakness is placed in the player's Threat Area. At least in the Core Set, in no case is the Treachery weakness added to the Investigator's hand as described in the bullet above. This seems to be supported by the instructions for Treachery (RR, p. 20).

• When a treachery card is drawn by an investigator, that investigator must resolve its effects. Then, place the card in its discard pile unless otherwise instructed by the ability.

So this leads me to believe that the reference to "player cardtype" is referring specifically to a signature weakness. I'm just not grasping that last bit "add it to that investigator's hand." I guess it makes sense for Wendy's "Abandoned and Alone" and Agnes's "Dark Memory" cards, but that's about all I can think of because the other three signature weaknesses get added to the Threat Area.

Can someone please help explain?

It only applies to player-card weaknesses (they will say events, assets or skills), treacheries and enemies are not added to your hand. You resolve treachery and enemy cards exactly like if you had drawn them out of the encounter deck, so resolving the revelation or spawning them at their spawn instructions (or engaged with you if there are no spawn instructions). So for the core set that rule is only relevant for Daisy's and Agnes's signature weakness if I'm not mistaken. Did this answer your question?

If we're just talking the core set, then the two signature weaknesses with player card types are actually those belonging to Daisy and Agnes - Wendy has a standard Treachery Weakness as a signature. Daisy's weakness is still an Asset, even though it has Revelation text which instructs you to put it into play when you draw it.

All the Basic Weakness cards in Core are either Treachery or Enemy, so you don't have to worry about them. However, once you start to broaden your Basic Weakness pool with cards from other cycles, you may find weakness with player card types amongst those you might draw...

And, of course, various characters - end even some items - may bring player card type weaknesses into your deck should you use them.