A shepherd owns the sheep and marks them. In some cases, sheep are branded. Although some sheep are branded, that's really not a popular thing because it damages the wool. Even if the brand is placed through the wool and into the hide of the lamb, the wool can overgrow it so the brand won't be seen. Today the ears are pierced with identification tags, but that's a fairly modern invention.

For thousands of years, shepherds around the world marked the ears of their sheep by notching their ears with a sharp knife. Each shepherd had his own distinctive notch for the ear of his sheep. If the sheep gather in a cluster, he can see even from a distance which ones are his.

I think all of this is a lot like being a Christian. For Christians are also those who admit to being owned and marked by Jesus Christ--sometimes marked painfully through suffering and difficulty. It must be painful for Jesus Christ to allow those marks to be burned, pierced, and notched into our lives.