Here's a discussion partially involving the precept substance (戒體) at H-Buddhism:Starts here
and look for the same subject throughout the month
. It elucidates a few things about it for sure, including its origin and some later developments.
The very useful series Buddhism in a Nutshell
summarises it as "those 'not-to-do' precepts". You can find a modern description in Zhenhua's In Search of the Dharma: memoirs of a modern Chinese Buddhist pilgrim
.Xuanhua seems to teach
the precept-substance as an actual thing, which appears to me as the common view in modern EA Buddhism:
"The Handbook of Bodhisattva Precepts for the Laity speaks of the unconditioned substance of precepts. The Dharma Master for the Bodhisattva Precepts will say: "All disciples of the Buddha, since all of you can bring forth these fourteen deep, vast vows, I now ascend the seat and bow on behalf of all of you, to respectfully invite the Buddhas and Bodhisattvas of the ten directions to transmit to you the perfect substance of the precepts." That is known as the precept substance. This is the most important time for you during the precept transmission. It's when you receive the substance of the precepts.
Everyone should read this section, so that when the time comes you can contemplate accordingly to receive this superior precept substance.
After producing faith and understanding, when we receive the precepts, we have to truly and sincerely place the six major and twenty-eight minor precepts in our minds, so that we can stop evil and avoid wrongdoing. We should put into practice the dharma of the precepts. We should not do what we are not supposed to do. Even when we dream, we should tell ourselves that it's not permissible to do such and such a thing. That power comes from the precept substance. We have that power if we have truly received the substance of the precepts. Once we have the precept substance, we will gain a kind of strength in our body, mouth and mind. There is a change in our conduct and demeanor. Even when we are dreaming, our thoughts and our behavior are not the same as before; this is known as the precept conduct. "He also says at another place
"Lay people who like to receive precepts must request them from an ordained monk. To transmit precepts means to give the precept substance to the preceptees. The person who transmits the precept substance to you must be a Bhikshu. In the Vinaya, a Bhikshuni is not permitted to transmit precepts."