I wasn't even aware of this rule until recently, but, from the FAQ:
(1.16) Multi-class Cards
A multi-class card is a card which bears multiple class icons instead of one,
and is a card of each of those classes. For example, a card with both a Rogue
() and a Guardian () icon is both a Rogue card and a Guardian card for
all purposes. Generally, a multi-class card can be included in an investigator’s
deck if that investigator has access to either of that card’s classes.
If an investigator has limited access to one of the classes on a multiclass
card and unlimited access to one of the other classes on that card, it will
still occupy one of the investigator’s limited slots unless their deckbuilding
options contains the word “other” in it (see 1.15, above).
As it turns out, this part only applies to the investigators with 3 subclass options who can take "up to 10 level 0-1 events/skills" of the chosen subclass. I've built a Carson deck demonstrating that arkhamdb doesn't enforce this rule:
On the Trail(1) is a Guardian card, but it's also a level 1 seeker event. Therefore, since Carson's deckbuilding doesn't contain the word "other", On the Trail(1) should occupy one of his 10 seeker slots, and this deck would be illegal.
I wasn't even aware of this rule until recently, but, from the FAQ:
As it turns out, this part only applies to the investigators with 3 subclass options who can take "up to 10 level 0-1 events/skills" of the chosen subclass. I've built a Carson deck demonstrating that arkhamdb doesn't enforce this rule:
https://arkhamdb.com/deck/view/3295686
On the Trail(1) is a Guardian card, but it's also a level 1 seeker event. Therefore, since Carson's deckbuilding doesn't contain the word "other", On the Trail(1) should occupy one of his 10 seeker slots, and this deck would be illegal.