At the end of this article, First Principles of Buddha Dharma, in the 'Summing Up' section, it says,
In the sense of Dharma being the 'teachings' of the Buddha, the teachings that explain the Path of Awakening, we find them composed in twelve types of scripture.
(12) Upadesa, concerning esoteric doctrines, the equivalent to Tantra ('tradition'), in which the Buddha appears in various archetypal forms, for the illumination of beings. These tantrika works are quite numerous, but not readily comprehensible except for those who have the correct key to their meaning.
What is that Key ?
What is the meaning of it?
Is it a secret of that tradition?