The righteous branch to which Jacob is referring is the family of Lehi. This doctrine began with the prophecy of Joseph of Egypt regarding a righteous branch that the Lord would restore to the gospel in the latter days, Joseph…obtained a promise of the Lord, that…God would raise up a righteous branch unto the house of Israel; not the Messiah, but a branch which was to be broken off, nevertheless, to be remembered in the covenants of the Lord that the Messiah should be made manifest unto them in the latter days, in the spirit of power, unto the bringing of them out of darkness unto light (2 Nephi 3:5). Jacob will make reference to this branch again in the allegory of the olive tree. In that passage, there is a branch which is planted in good ground and produces part wild fruit and part good fruit. See Jacob 5:25. As Jacob makes clear, the Nephites understood that they were part of scattered Israel. Their writings concentrate on the promises of the Lord to the house of Israel because those promises apply directly to them and their descendants.