From a quote by "Claire" in the film Elizabethtown

The significant other of the substitute person may not "have enough time" or "be too busy" for a relationship, or they may only be interested for the purpose of sex, money, status, or general need of someone to talk to during times of stress. The substitute person generally won't get the emotional fulfillment from the experience that he/she may crave, and will feel unimportant yet somehow will continue on out of hope.

This can only end in tears for the substitute person.

Bro since I got dirtpilled on Tuesday I have made sooo many worm friends and made them soo many little houses to get married in. Me? Lonely? No you’re the lonely one u lawn owning freak

"You and I have a special talent," Claire says to Drew. "And I saw it immediately. We're the substitute people. I've been the substitute person my whole life. I'm not an Ellen a co-worker Drew was into. I never wanted to be an Ellen. And I'm not a Cindy either…I like being alone too much. I mean, I'm with a guy who is married to his academic career. I rarely see him and I'm the substitute person there. I like it that way. It's a lot less pressure."

"Since Anne just broke up with Joe, Emily is Joe's substitute person."