Some card abilities say "Unlimited", some give a limit "Once per turn", "Twice per phase", etc. What about abilities that do not have an accompanying limit in the text? How often may those be used? It's a little confusing to have some cards be "unlimited", others with a limit, and then others with neither limit nor lack of limit.

32 minutes ago, zenandhonor said:

Some card abilities say "Unlimited", some give a limit "Once per turn", "Twice per phase", etc. What about abilities that do not have an accompanying limit in the text? How often may those be used? It's a little confusing to have some cards be "unlimited", others with a limit, and then others with neither limit nor lack of limit.

Unless specified otherwise each card can use its abilities once per turn. There will be some cards like Kitsuki Investigator that have an additional clause that says Max 1 per conflict which means regardless of how many investigators are in the fight only one of them can activate its abilities per side.

What you want is the Rules Reference document.

https://images-cdn.fantasyflightgames.com/filer_public/95/ea/95eab8a3-1149-4398-bcad-40a2d87be600/l5c01-online_rulesreference_v16.pdf
https://fiveringsdb.com/rules/reference (HTML version)

Under Triggered Abilities (and Reactions and Interrupts), it states that unless otherwise specified, each action can only be triggered once per round.

https://fiveringsdb.com/rules/reference#triggered-abilities

The "otherwise specified" is the card texts that you have found (Limit twice per round, Unlimited, Max 1 per conflict)
There should be no card with (Limit 1 per round), as that is essentially the default.

The Limit and Max section also bear reading.
https://fiveringsdb.com/rules/reference#limit-x-per-period
https://fiveringsdb.com/rules/reference#max-x-per-period

Plenty of reading for you to do, but we're happy to answer questions.

Thank you, guys!

On a related note, it is my understanding that Interrupts and Reactions fall under the Abilities category. In that assumption, can they still be used during a Window of Opportunity once the respective player has passed, or not?

In the same way that players alternate taking Actions during an Action Window, players alternate during an Interrupt/Reaction Window.

Let's take a common Triggering Condition (TC): Breaking a Province

There is an Interrupt Window prior to the TC resolving. First player can Interrupt or pass. Then go back and forward until both players consecutively pass.

Then the province breaks.

Then there is a Reaction Window, with first player going first.

After this Reaction Window, you go back to the next player's action, or the next step in the Framework.

No offense, but I had thought that Interrupts and Reactions could be played in response to appropriate Actions. For instance a player takes their turn to use an Action during the Window of Opportunity and the other player plays an appropriate Interrupt or Reaction in response and not necessarily as part of the alternation of Action Opportunities.

17 minutes ago, zenandhonor said:

No offense, but I had thought that Interrupts and Reactions could be played in response to appropriate Actions. For instance a player takes their turn to use an Action during the Window of Opportunity and the other player plays an appropriate Interrupt or Reaction in response and not necessarily as part of the alternation of Action Opportunities.

That is correct. I think Shunsuke was being a little unclear in his description as it would be Player one takes an action. This creates an interrupt/reaction window for both players. After Player 1's action resolves player 2 has the option of perform an action which creates another interrupt/reaction window. Play than alternates back and forth between both players until they both pass at which point you move onto the next step in whatever phase you are in.

The only time where passing would lock a player out from initiating an action is the Dynasty phase as once you claim your passing fate you may no longer initiate an action. However you are still able to play reactions and interrupts to other players initiating an action as they are not action in and of themselves.

Okay, that is MUCH clearer. Thank you guys, both of you.

So once the Interrupt/Reaction Window (wherein only Interrupts and Reactions can be used, I presume) it returns to the normal Window of Opportunity?

8 minutes ago, zenandhonor said:

Okay, that is MUCH clearer. Thank you guys, both of you.

So once the Interrupt/Reaction Window (wherein only Interrupts and Reactions can be used, I presume) it returns to the normal Window of Opportunity?

Yes with what ever limitations may have been imposed on that window