Charles Ellicott Commentary Leviticus 4:23

"if his sin, wherein he hath sinned, be made known to him, he shall bring for his oblation a goat, a male without blemish." — Leviticus 4:23 (ASV)

Or if his sin. —This means, if he fails to see it himself, his sin is pointed out to him by another person.

A kid of the goats. —The expression used here, sâêr, properly denotes the rough, shaggy-haired male goat. It is distinguished from athud (literally, ready, vigorous), which appears with it (Numbers 7:16–17; Numbers 7:22–23) and is also translated "goat" in terms of its age.

The sâêr, or the shaggy, longer-haired male goat mentioned here, is the older buck goat whose hair has grown long with age, while the athud is the same animal but younger and more vigorous.

Hence, the former (sâêr) was never killed for food or used for burnt offerings or thank offerings at the festivals (Leviticus 16:9; Leviticus 16:15; Leviticus 23:19; Numbers 28:15; Numbers 28:22; Numbers 28:30; Numbers 29:5; Numbers 29:11; Numbers 29:16), nor at the consecration of the priests and sanctuary (Leviticus 9:3; Leviticus 9:15; Leviticus 10:16).

The latter (athud), however, was killed for food (Deuteronomy 32:14; Jeremiah 51:40). Consequently, like the bull, the ram, and the lamb, it was regularly presented as burnt offerings and thank offerings (Numbers 7:17; Numbers 7:23; Numbers 7:29, and following; Isaiah 1:11; Isaiah 34:6; Ezekiel 39:18; Psalms 50:9, 50:13; Psalms 66:15).

Thus, it becomes clear that the first distinction in the sin offering for a ruler is that he is to bring a long-haired male goat, and not a bull.