Nasai

(1) "Wavy": It is possible that they were naturally wavy. It is also possible that, due to their length, they became curled. This commonly happens with long hair.

(2) Between the ears and the shoulders: It appears that you (sallallahu alayhi wa sallam) would trim your hair to the level of the lower part of the ears. When it grew long enough to reach the shoulders, you would then cut it. It can also be said that when you bowed your head, your blessed hair would seem to be at the level of the ears, and when you raised your blessed head, it would touch the shoulders. In normal circumstances, it would remain between the ears and the shoulders. And Allah knows best.

(3) In both cases, it indicates the cutting of hair.

(4) From this blessed hadith, it is also understood that the ruling for the beard and the hair of the head are separate. Cutting and shaving the hair of the head are both permissible, whereas cutting and shaving the beard are both impermissible and unlawful acts.

(5) The Messenger of Allah (sallallahu alayhi wa sallam) was a masterpiece of creation, therefore, semi-curly hair itself is a sign of beauty and elegance, just as the Prophet (sallallahu alayhi wa sallam) had.

Hadith Commentary:
(1)
There are three types of hair:
➊ Wafrah:
These are the hairs that reach up to the earlobes.
➋ Jummah:
These are the hairs that are between the shoulders and the ears.
➌ Limmah:
These are the hairs that reach up to the shoulders.
All three types regarding the hair of the Messenger of Allah (sallallahu alayhi wa sallam) are narrated in the hadiths.
This is according to different times and different circumstances.

(2)
In reality, the hair of the Prophet (sallallahu alayhi wa sallam) was wavy.
When he would comb his hair, it would reach up to or near the shoulders, and after some time had passed, his hair would become wavy and reach up to the earlobes.
Whoever saw his hair after it had been combed reported that his blessed hair was up to the shoulders, and whoever saw it in the other state narrated that it was up to the earlobes; that is, each narrator reported what he had observed.
And Allah knows best. (Fath al-Bari: 10/439)

Hadith Gloss:
Vocabulary of the Hadith:
Ja‘d:
Curly (hair),
Sabit:
Straight (hair).
Rajil:
Combed.

Hadith Commentary: Benefits and Issues: According to Mulla Ali Qari, the Prophet Muhammad (sallallahu alayhi wa sallam) shaved his head on the occasion of ‘umrah and hajj. After that, the hair would begin to grow; when the time since shaving the head was recent, it would reach up to the middle of the ears. Then, as it gradually grew, it would reach the earlobes, then reach between the ears and the shoulders, and then fall upon the shoulders. Then, according to some, when he had his hair trimmed, it would be up to the middle of the ears, and then it would continue to grow gradually. In this way, at different times, there would be different conditions.

English translation:

Up to half of the ears—this is not contrary to the preceding narrations. At the time of cutting, they would be equal to half of the ears, and then they would grow longer.

Regarding the hair of the Prophet (sallallahu alayhi wa sallam), three words have appeared in the hadith: al-jumma, al-limmah, and al-wafrah.

➊ Al-jumma: Locks of hair hanging down to the shoulders and upper arms.
➋ Al-limmah: Locks of hair grown beyond the earlobes.
➌ Al-wafrah: Hair reaching up to the ears, that is, hair touching the ears is called wafrah.

The blessed hair of the Messenger of Allah (sallallahu alayhi wa sallam) would be in different forms at different times. The mentioned hadith describes one of these forms.

Benefits and Issues:

➊
The meaning of "straight" is that they were not extremely curly, rather they had a slight wave.

➋
When his hair was cut, it would reach the earlobes, and when it grew longer, at times it would reach the shoulders.

➌
On the occasions of Hajj and Umrah, the Messenger of Allah (sallallahu alayhi wa sallam) would have all the hair of his head shaved off.