This list is incomplete and shall be updated and expanded as more related vanilla C# code details are found out and/or as more C# functionality are added.

INITIATOR_ and RECIPIENT_

These prefixes are largely used to refer to pawn-related data. Here's a partial list of elements you can write just after these prefixes.
Most of the entries are self-explanatory, and those I believe to be the main ones are shown in a larger font.

age / chronologicalAge

bestSkill / worstSkill

Example: INITIATOR_bestSkill==animals

faction / factionName

Examples for "faction": Empire, PlayerColony

gender

Possible values: Male, Female.

higherPassion

Returns the higher passion of the pawn. In case of ties, the choice is random among the tied skills.

jobDefName

It's the generic name of a particular job being executed by the pawn. A more specific ID is contained in the related jobText parameter.
I don't have the full list - what I know I deduced during debugging sessions.
Here's a very partial list of jobDefName options:
BuildRoof, Clean, CutPlant, FinishFrame (build structures), GotoWander, HaulToContainer, HaulToCell, Hunt, Ingest, LayDown (going to lay down/laying down), Pray, Wait_Wander, Research, Skygaze, SocialRelax, Sow, StandAndBeSociallyActive, VisitSickPawn

mood

nameDef

nameFull

opinion

It's the [-100,100] opinion the INITIATOR has about the RECIPIENT.

pronoun / possessive / objective

For a female pawn, these equal to she, her and her respectively.
For a male pawn, these equal to he, his and him respectively.

thoughtDefName

It refers to the DefName of a pawn's thought. Listing all the possible pawn's thoughts (even if we just consider vanilla only) would be too long and it's currently out of the scope of this wiki.
Nonetheless, if you wish to find out more details, you can open the XML documents available in the [...]\RimWorld\Data\Core\Defs\InteractionDefs game folder. You can check other Defs sub-folders, too, for anything related with the interaction keyword.
For each thoughtDefName there is a thoughtText associated, which refers to the description of the thought.
Example: INITIATOR_thoughtDefName==SleptOnGround

royalTitle0

I'm quite sure it's the DLC royal title. I am sure one option is "freeholder".

title

You can find the whole titles list in the Backstories.xml file. There is no universal path for this, but it's probably compressed in one of the language-specific .tar files.
You could also write XML lines for a specific custom title, and then give this custom title to pawns in your game, so that these will use the title-specific lines. For instance, you could create a specific character called "Pulcinella" and write specific lines for it. Then, while playing, you select a pawn and set its title to "Pulcinella" - this way, the pawn will use the specific lines you wrote.

trait

The full traits list (at least for the vanilla ones) is available here. You can use the names listed there, but with the first letter in lower case.
Example: RECIPIENT_trait==wimp

[skill]_level

The full vanilla skill list is: shooting, melee, construction, mining, cooking, plants, animals, crafting, artistic, medical, social, intellectual.
Example: RECIPIENT_cooking_level>=5

[skill]_passion

The possible values are: None, Minor, Major.
Example: INITIATOR_animals_passion==Minor

seated

Checks if the pawn is seating on some furniture. Possibile values are Yes and No.

VANILLA WILDCARDS

They must be enclosed in [] brackets. A partial list is: AdjectiveAngsty, AdjectiveAny, AdjectiveBadass, AdjectiveFriendly, AdjectiveLarge, Animal, AnimalGroup, Apparel, Color, Community, Enemy, Gore, Mechanoid, PersonJob, Quantity_adjphrase, TerrainFeature, TreeType, Vegetable, VerbFriendly, Weapon

Other parameters

DAYPERIOD

Possible values are: morning, afternoon, evening, night.

HOUR

Possible values are integers between 0 and 23.

NEAREST_

This prefix is used with a postfix to return the specified world object type that is nearest to the initiator pawn. Possibile postfixes for now are: art, plant.
Example: NEAREST_plant returns the nearest plant type object nearest to the initiator pawn.

OUTDOORS

Checks if the initiator pawn is outdoors. Possibile values are Yes and No.

TEMPERATURE

WEATHER

Possible values are: clear, fog, rain, dry thunderstorm, rainy thunderstorm, foggy rain, hard snow, gentle snow, flashstorm.