Ibn Majah

Benefits and Issues:
➊
On the occasion of Hajj, shaving the head is superior.
The Messenger of Allah (sallallahu alayhi wa sallam) also had all of his head hair removed. (Sahih al-Bukhari, Book of Hajj, Chapter: Shaving and Shortening at the Time of Releasing from Ihram, Hadith: 1726)

➋
For women, shaving the head is not permissible, (Jami‘ at-Tirmidhi, Book of Hajj, Chapter: What Has Been Reported Regarding the Dislike of Shaving for Women, Hadith: 914/915; and Sunan Abi Dawud, Book of Manasik, Chapter: Shaving and Shortening, Hadith: 1984/1985).
It is sufficient for them to cut a small portion of hair from the head.

Hadith Commentary:
That is, al-Layth is in doubt as to whether the Prophet (sallallahu alayhi wa sallam) supplicated once or twice for those who shaved their heads, and most narrators agree with the narration of Imam Malik that the Prophet (sallallahu alayhi wa sallam) supplicated twice for those who shaved their heads, and on the third time included those who trimmed their hair as well.
In the narration of Ubaydullah, it is mentioned that on the fourth time, those who trimmed their hair were included.
In any case, what is derived from the hadith is that shaving the head (halq) is superior to trimming the hair (taqsir). Imam Malik and Imam Ahmad say that the entire head should be shaved, while according to Imam Abu Hanifah, shaving a quarter of the head is sufficient, and according to Imam Abu Yusuf, shaving three hairs is sufficient.
Some of the Shafi‘iyyah have considered shaving even one hair to be sufficient, and women should trim their hair; it is forbidden for them to shave their heads.
(Wahidi)
The incident of shaving or trimming the hair is related to the Farewell Pilgrimage (Hajjat al-Wada‘) and also to Hudaybiyyah, when the people of Makkah prevented the Prophet (sallallahu alayhi wa sallam) from performing ‘umrah. The Prophet (sallallahu alayhi wa sallam) performed shaving and sacrifice right there in the plain of Hudaybiyyah. Even now, for those who are prevented from Hajj or ‘umrah on the way, this is the ruling for them.
Hafiz Allamah Ibn Hajar (rahimahullah) says:
“As for the reason for repeating the supplication for those who shaved their heads during the Farewell Pilgrimage, Ibn al-Athir said in al-Nihayah: Most of those who performed Hajj with the Messenger of Allah (sallallahu alayhi wa sallam) did not bring sacrificial animals with them. So when the Prophet (sallallahu alayhi wa sallam) commanded them to terminate their Hajj and perform ‘umrah instead, and then exit ihram and shave their heads, this was burdensome for them. However, since obedience to the command was necessary, they found trimming easier upon themselves than shaving, so most of them did that. Thus, the Prophet (sallallahu alayhi wa sallam) gave preference to the action of those who shaved their heads because it was more apparent in fulfilling the command.” (That is, the Prophet (sallallahu alayhi wa sallam) supplicated abundantly for those who shaved their heads because most of the pilgrims with him had not brought sacrificial animals. When the Prophet (sallallahu alayhi wa sallam) commanded them to terminate their Hajj, perform ‘umrah, exit ihram, and shave their heads, this was difficult for them. Yet, since obedience to the command was necessary, they found trimming easier than shaving, so most of them did that. Therefore, the Prophet (sallallahu alayhi wa sallam) gave preference to the action of those who shaved their heads, as it was more manifest in fulfilling the command.)
It was also the custom of the Arabs to grow their hair long and derive adornment from it, and shaving the head was uncommon among them. They also considered long hair a sign of fame among non-Arabs and regarded imitating them as a means of gaining fame for themselves.
Therefore, most of them considered shaving the head to be disliked and preferred to suffice with trimming the hair.
From the above hadith, it is also established that supplicating for those who are prepared to do the best of deeds is permissible, and it is also established that one may request supplication for those who act upon a less preferred option. It is also established that trimming (taqsir) is sufficient in place of shaving (halq), but shaving is better.

1⎖:
From this hadith, it is understood that shaving (halq) is superior to shortening (taqsir) the hair.

1979. Commentary: On the occasion of Hajj, it is preferable for men to shave their heads with a razor. This ruling does not apply to women; they should trim a small portion of their hair.

Hadith Authentication: [واخرجه البخاري 1727، و مسلم 1301/317، من حديث مالك به]
Jurisprudential Explanation:
➊ Shaving or cutting the hair of the head at the conclusion of Hajj and Umrah is an act of worship and among the rites; apart from this, it is permissible at all times, and to shave the head considering it an act of worship and virtue (outside of Hajj and Umrah) is a sign of the Khawarij. Those who restrict shaving the head only to Hajj and Umrah and consider it forbidden or impermissible for others, their view is incorrect.
➋ After Hajj and Umrah, shaving the head is more virtuous than trimming the hair.
➌ Qasim bin Muhammad bin Abi Bakr rahimahullah, when he would enter Makkah at night for Umrah, would perform the circumambulation (tawaf) of the House of Allah, perform sa’i between Safa and Marwah, and would delay shaving his head until morning, but would not go home before shaving his head. [الموطأ 1/395 ح913 وسنده صحيح]

Benefit:
From this hadith, it is established that shaving the head at the completion of Hajj and Umrah is superior, and on this occasion, trimming the hair with scissors is also permissible.