"Do you have source for this? I am trying to look but could not find anything that would define "training (unlimited)" vs "education assistance (capped)". "

You're asking about Apples and Puppies.

Do I need to worry about a limit when I deduct the office electricity expense? What about if I reimburse an employee for mileage under An Accountable Plan?

Different types of expenditures, one of which has qualifications and limitations.

Paying a trainer is an operating expense.

Reimbursing an employee for their tuition, which they get at a qualified/certified/accredited institution (and not from the employer) is different. That's a Benefit.