Genesis 4:2

And she added to bear from her husband Adam his twin, even Habel. And Habel was a shepherd of the flock, but Kain was a man working in the earth.

ויהי הבל רועה צאן, Hevel chose this vocation as it required more intelligence and involved one’s mental activities more that farming.

וקין היה עובד אדמה, as a result of each having his own vocation, they each brought different offerings to G’d.

ותוסף ללדת את אחיו את הבל, the construction of ותוסף ללדת instead of ותהר עוד ותלד, “she again became pregnant and gave birth,” supports the words of our sages in Bereshit Rabbah 22,2 that the two brothers Kayin and Hevel were born as twins. The birth was additional, not the pregnancy. Our sages added further that not only were these two brothers born as twins, but that a twin sister was born at the same time to each of them. This also is in line with a statement by Rabbi Eleazar ben Azaryah in the same Midrash that on that day three miracles occurred. 1) Adam and Chavah had been created on that day. 2) They had engaged in marital intercourse on that same day. 3) They produced twin children on that same day.
Rabbi Yehoshua ben Korchoh phrased it as follows: “2 human beings united on the bed and eight descended from the bed.” He referred to Kayin and his twin sister and Hevel with two twin sisters. [in my count this makes only 7. Actually, in my version of Bereshit Rabbah, it says that “seven” descended from that bed. Ed.] The Torah fails to mention why Chavah called her second son Hevel. [In fact, the Torah does not say that it was Chavah who named Hevel. Ed.]
There is nothing unusual in the Torah not providing the reason for the name Hevel, as there are hundreds of names which have not been explained in the Torah. None of the names of the people in the generations between Sheth and Noach have been explained. Perhaps, the reason why Chavah called the second son Hevel is an allusion to Psalms 62,10 כי הבל בני אדם כזב בני איש, “men are mere breath; mortals illusion.” Perhaps Chavah expressed her realisation that even the good found in this material world may prove short-lived, illusory, as her brief sojourn in Gan Eden had taught her. Seeing that both her husband’s and her own life had become very difficult compared to what it had been like, she showed that she had assimilated this truth as being something she had to bequeath to her offspring now that she had children. It is immaterial in this connection if Hevel was named by her or by her husband.