Who are the hundred and forty four thousand? Are they appointed by a General Authority?
The Doctrine and Covenants may shed some light onto your question.
As you know, the One Hundred and Forty Four Thousand comes from a scripture in Revelation 7:4, where John states, that in a vision, “I heard the number of them which were sealed: and there were sealed an hundred and forty four thousand of all the tribes of the children of Israel.” John continues to state that twelve thousand of these will come from each tribe. See Revelation 7:4-8.
Joseph Smith asked almost the exact same question as you – who are these one hundred and forty four thousand? The Lord gave him an answer in Doctrine & Covenants 77:12. In response to the question “What are we to understand by sealing the one hundred and forty-four thousand, out of all the tribes of Israel – twelve thousand out of every tribe?”, the revelation of Joseph Smith states that
“We are to understand that those who are sealed are high priests, ordained unto the holy order of God, to administer the everlasting gospel; for they are they who are ordained out of every nation, kindred, tongue, and people, by the angels to whom is given power over the nations of the earth, to bring as many as will come to the church of the Firstborn.” D&C 77:11-12.
Church educational manuals have elaborated:
“‘the number 144,000 mentioned in Revelation 7:4-8 is the number of ordained high priests out of the twelve tribes of Israel who will assist others in their quest for exaltation. it is not, as some people believe, the total number of people who will be exalted.” See New Testament Seminary Teacher Manual, Lesson 154: Revelation 6-11, Part 1
As a final note as to who these men are, Church educational materials state that the 144,000
“had done with their lives what ancient Israelites did with their crops – they gave the Lord the first and best of their obedience. Thus they were the “firstfruits” of the Lord’s people on Earth.” See New Testament: Student Study Guide, “Revelation 14: Harvest Time on Earth“
So to recap, here is what we know about the 144,000:
1) They were extremely righteous men during their time while alive.
2) They were ordained by angels to whom is given power over the nations of the Earth.
3) Their purpose is to come down in the end of times and assist people to obtain exaltation.
4) They are not the only people who will attain exaltation.