Beyond the point that custom fields are pretty much always advanced, there has to be a point at which a term is so fundamentally non-specific that it cannot be subject to trademark.
Single English words cannot be trademarked. However, if you string two of them together, and demonstrate that you are actively using the phrase in commerce, you can get the phrase trademarked for use in a particular domain, e.g. computer software.
Three words is even better.
Note that a tractor manufacturer could still trademark "Active Custom Fields" for agricultural equipment, because it would not be confusingly similar to the "Active Custom Fields" software.
Also trademarks have to be renewed every 5-10 years and you must show that you are actively using it.
I think the key is that these are trademarked for a particular domain. If I tried to sell software as Alphabet I'd be shut down, but my kid's teacher doesn't need a license to teach the alphabet.
Beyond the point that custom fields are pretty much always advanced, there has to be a point at which a term is so fundamentally non-specific that it cannot be subject to trademark.
I just think it's bloody madness sometimes.