There are two traditions in sutra: one is the common, or typical, sutra tradition that focuses mainly on the teachings of the Buddha in the first and second turnings of the wheel of Dharma; the other is the uncommon sutra tradition. Many of the scriptures in the Buddha’s third turning of the wheel of Dharma dwell on the six perfections and the four methods of guidance that underlie the vast activities of the bodhisattvas, but there are also ten quintessential sutras that place a great deal of emphasis on luminosity of tathāgatagarbha, the luminous mind of the buddhas that is inherently present in each being; we call these ten sutras in the third turning the uncommon sutras. The view in uncommon sutra is entirely different from the view in common sutra, but very similar to that in tantra, even if it is not as clearly stated. For this reason, these ten sutras are said to be “half sutra half tantra.”

~Depicted from GATEWAY FROM VAJRAYANA PATH - Vajrayana Terminology