The Definition for Scripture:
D&C 68: 1-6:
1 MY servant, Orson Hyde, was called by his ordination to proclaim the everlasting gospel, by the Spirit of the living God, from people to people, and from land to land, in the congregations of the wicked, in their synagogues, reasoning with and expounding all scriptures unto them.
2 And, behold, and lo, this is an ensample unto all those who were ordained unto this priesthood, whose mission is appointed unto them to go forth —
3 And this is the ensample unto them, that they shall speak as they are moved upon by the Holy Ghost.
4 And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.
5 Behold, this is the promise of the Lord unto you, O ye my servants.
6 Wherefore, be of good cheer, and do not fear, for I the Lord am with you, and will stand by you; and ye shall bear record of me, even Jesus Christ, that I am the Son of the living God, that I was, that I am, and that I am to come.
This scripture actually feels like SCRIPTURE. Not only does it make good logical sense, but the Holy Ghost also says that it is true. The next scripture feels exactly the same to me as well.
D&C 1: 36-39:
36 And also the Lord shall have power over his saints, and shall reign in their midst, and shall come down in judgment upon Idumea, or the world.
37 Search these commandments, for they are true and faithful, and the prophecies and promises which are in them shall all be fulfilled.
38 What I the Lord have spoken, I have spoken, and I excuse not myself; and though the heavens and the earth pass away, my word shall not pass away, but shall all be fulfilled, whether by mine own voice or by the voice of my servants, it is the same.
39 For behold, and lo, the Lord is God, and the Spirit beareth record, and the record is true, and the truth abideth forever and ever. Amen.
The Holy Ghost is God. Whenever God speaks, it is scripture. Therefore, whenever God or the Holy Ghost is speaking through his Prophets or Servants, then it is scripture.
With enough scripture study, practice, fasting, and prayer, you can actually learn to discern when the Prophets are being moved upon by the Holy Ghost to speak scripture, and when they are simply giving their personal opinion as mortal men. Much of their personal opinion has value, but it doesn’t carry the same weight as it does when God is speaking to us through them.