When a person casts a corporeal Patronus for the first time, a mark in the shape of their Patronus appears on their soulmate's body. Severus never needed a soulmate mark to tell him who to love, though...

"If I give my heart to you
I must be sure
From the very start that you
Would love me more than her"

Severus hadn't cast a Patronus since Lily died.

The first time he'd met her, she'd been a scrawny, gangly thing with hair that blazed like the sun. He'd loved her from that first moment, so caught up in her orbit that he barely had any patience for anyone else. He was so convinced they were soulmates that he began practicing the Patronus Charm as soon as he got his wand. Always eager to learn more magic, Lily had insisted on practicing with him. They finally pulled it off the day before they left for their first year at Hogwarts, laughing in delight as their nearly identical corporeal does danced around them.

Severus had expected them to each manifest a soulmate mark after that, but they hadn't. Lily hadn't been bothered by it, but Severus was shattered. He tried to convince himself that it didn't matter. There was no rule that you had to love your soulmate, no guarantee that you'd even get a soulmate mark. But then they got to Hogwarts where James Potter bragged about having a soulmate mark that looked exactly like Lily's Patronus, and that had been that.