A YouTube video by a Japanese FFBE player inspired me to make this thread, as I am sure there are some people who don't currently know this. For those of you who also speak Japanese, I will include a link at the bottom of this post.

With the release of Dark and Fire Veritas we now have access to TMs that can provide up to 60% in passive attack bonuses. While these do happen to be on 5★ base units right now, it's not to far fetched of an idea to consider similar abilities coming from future 4★ base units, or even events and trials; that, and future 5★ base units may provide even greater passive increases as well.

Why is this important?

There is a limit to how much benefit you receive from passive attack increases. That limit is 300%. Once you reach 300% in total passives that increase attack, anything over that mark won't take effect. And yes, this includes innate passives such as Orlandu's 30% increase to attack or Dark Veritas' Greatsword Mastery.

Let's take Dark Veritas as an example, and for the purposes of explanation let's also assume you have 4 of his TMs. His passive increases to attack would look like this:

• 六盟傑の誇り・闇 = 60%
  
• 六盟傑の誇り・闇 = 60%
  
• 六盟傑の誇り・闇 = 60%
  
• 六盟傑の誇り・闇 = 60%
  
• 攻撃 20% = 20%
  
• 大剣の極意 = 50%
  
• Total = 310%

At this stage he would have 310% in total passive attack modifiers; however, only 300% of that would actually take effect, meaning that 10% would be wasted. This example just takes into account materia and innate passives, but passive attack on equipment also counts. In the above scenario, if you were to give Dark Veritas Dual Wield via the Genji Gauntlets then his passive attack modifier would rise to 320%, in which case you would then be wasting 20% worth of passive attack increases.

Important note: The limit I talk about is with regards to passives that are considered abilities; essentially, if it shows up under the abilities section of your character, regardless of its origin, then it counts towards this limit. You can go above the limit via in-battle buffs.

Important note 2: I'm not sure if this will also be the case with magic but since there is a limit to how much passive attack you can receive benefit from I can only assume that magic will be the same as well.

Edit 1: It would appear that this 300% cap also applies to other passive statistics such as magic as well, not just passive attack. Thanks /u/Nazta and /u/akairyuvn for following up on this!

Video link