Table 2.

Representative quotes.

Design component and affordance		Young adults’ experiences and practices
Credibility
	Promise		Trust in app intent: It made it seem more legit. Like it was actually something making you try to quit smoking instead of, you know, maybe the other ones have ads in them and they try to make money off them. But this one clearly isn’t. It’s a little more genuine, you know? (male, nonsmoker, P24)
Social support
	Constrained identity protection		Feelings of vulnerability: And if you don’t [quit] like people might get on you and nag you or be disappointed. I don’t need that. I’ll be disappointed in myself, that’s enough. (male, smoker, P21)
			Selective posting: You know, I’m gonna kinda keep it to myself and work away at it. And then, once I have quit for good, then maybe I can go and say like, this is where I’m at this point in the app. I’ve quit smoking completely, or it’s been 100 days or whatever the case may be if I wanna share my milestone or something like that. But [its]…nothing I would use on a regular basis for sure. (female, smoker, P26)
	Inhibited competition with others		Discouraging and divisive: Someone whose successful and quit smoking isn’t any better than someone that’s struggling with it. Like, no, I didn’t-I don’t like that aspect… it just makes someone feel bad. (male, nonsmoker, P10)
	Constrained co-participation		Unhelpful: I tried to do the quit buddy thing but the people that I was having as quit buddies were not as serious about quitting as I was so. (male, nonsmoker, P17)
Task support
	Visibility of the benefits of quitting		Perception shift – health implications became relevant: Yeah, I would check that one a lot because it would keep coming up and like showing I’m this close to being…back to like a nonsmoker for this aspect of my health, or this aspect...I guess it was more of a motivation to quit because some of them it takes like years to get back to a non-smoker health state and that just really made me think about what I was doing. (female, smoker, P14)
	Documentation of smoking behavior		Raised awareness of smoking habit: I thought I knew how much I was smoking but this [smoke button] gave me a reality check….it was neat to see how much I was actually smoking. (male, smoker, P23)
	Mapping out smoking behavior		Irrelevant information: For me, it’s definitely not geographic triggers that make me wanna smoke. It’s more like…like day-to-day triggers of either stress, or commuting traffic, and like bad news. You get bad news broken on you or you had a bad day it’s like, yeah I’d be more tempted to pick up that cigarette. Those type of things. More than the, oh like I’m located at 24 Sussex here and, I have a temptation. (male, smoker, P3)
	Journaling about smoking behavior		Unhelpful (men): No [I wouldn’t put in my triggers]. I would just put the craving in but the triggers, my main triggers were coffee, which I have quite a few a day, friends, drinking, after a meal.. It’s the first thing I wanna do. (male, smoker, P4)
			Proactive about triggers (women): I loved…how you could go in and track [journaling] what your triggers were so I could actually start to notice my triggers and stay away from them. I’m craving when I’m with friends drinking, or you know, my son’s acting up, and stuff like that. So I could see what was actually causing me to want to smoke and I could actually try and change them. [For example], it was my friend’s birthday party and I knew we were all going to be in the same house and she smokes in her house. So I had to come prepared. I brought gum, I brought mints, I brought everything I could think of because that was one of my main times when I smoke a lot…when I’m drinking. (female, smoker, P25)
	Weening from smoking		Procrastination: I actually think that that’s [quitting abruptly instead of weening off smoking] the way that it has to be done….Like it’s almost like exercising and diet. You can’t say you’re gonna start next week. It’s really like right now or never…. Like that’s one thing that I think was the flaw in CTC. (female, smoker, P28)
	Counseling through cravings		“Backpocket” support: I didn’t personally call [the quit line] but it was just—I think it was more of just knowing that like, if at any time, if I didn’t have someone to talk to or if there wasn’t a feature on the app that I could use, [the quitline] was kind of like in my back pocket right? Like if I absolutely 100% needed to make a call, I could. (male, nonsmoker, P17)
Dialogue support
	App reminding		Irritating: I completely ignored them [notifications]. Actually, I’m pretty sure I had the notifications that were from the app all turned off. It just felt like a pop up, like another thing for me to click close on throughout the day. I completely paid no attention to it. (male, nonsmoker, P10)
			Motivation to quit (men): I found it [notification reminders?] was almost like having my girlfriend there, in a good way. So you’re like, oh I haven’t done this in two days, I didn’t even realize, but my phone just reminded me. Better keep it going. (male, smoker, P3)
	Recognition of ability		Confidence boost: It [what??] was just a reminder just to say like how good you’re doing. And if there’s nobody physically around you to be like, oh you’re doing such a great job, then [the app] did it for you. Like yeah, keep up the good work, you’ve been this many days without a craving or whatever else. It was kinda like a motivational boost. (female, smoker, P11)
	Visibility of quit efforts		Motivation to quit: It [what?] was like a visual of my day of smoking. And every day, you’d look at it, it went down and down and down, like it got better every day. So it was like a motivational thing to just look, like positive reinforcement. (female, smoker, P11)
			Discouragement: If you're having a bad day or a couple of bad days, seeing it on [the app] as a reflection [of your bad days] just like kicks you in the face even more, you know? (female, smoker, P22)