What does it mean when a girl with a boyfriend gives you her email instead of number?

This female friend I have a crush on, has a boyfriend. The girl and I are friends, pretty good friends I think, at least "acquaintance friends", seeing each other a lot at the restaurant, she's a waitress at. My favorite waitress. She's real nice to me, in a lot of ways, sweet, making me feel good, and accepting of my problems. I've known her for a couple years. We started as customer-waitress, then friends, then seemed to become better friends, when I told her I almost died in my trailer fire, a year and half ago. Something seemed to "switch" in her when she heard that, she did a double take, and looked at me intently, like she was completely shocked to hear I almost died. Ever since then, she's been very sweet, kind, helpful. She laughs at all my jokes, stays in our convos with each other, until she HAS to move to her next duty, eye contact, bright smiles, seemed to be happy to see me, especially after I've been gone awhile, almost missing me. I started falling for her sometime ago, over that period of time. I don't know if she has any feelings for me or not, even slightly, and would date me if she WAS single, but I'm hoping. But the thing is, for awhile now, I wanted to tell her how I felt, just to get it off my chest, at least, besides trying to find out if she has any feelings too, any at all. I wanted to tell her, yesterday, then decided to try to get to know her a little better, don't know much about her. I always talk about myself, problems, dreams, goals and future ambitions, to impress her pretty much, and so she would know me better. She's always encouraging me, advising, giving tips, liking my ideas. Or at least seems to. So anyway, She HAS to know I like her some. I ACCIDENTALLY fell for her, it wasn't by choice, it just happened. But she HAS to know, because of the attention I put on her...just can't help it, guess I'm addicted. She makes me feel great, being around her, and I seem to make her feel good too, when I joke with her, say/do nice things for her, or her daughter. So anyway, decided to try to get to know her better, work up my nerve to tell her how I felt, regardless of what she'd say, just to FINALLY let her know. So I asked if I could get to know her, she said sure, what would I like to know, she's an open book. I asked a couple basic questions (found out her b'day is in Sept, thought I might give her a card, if OK with her, just to make her happy. She said she's turning 32, and remarked she's getting old. I joked, to make her feel better, hey, you think that's old? I'm 43, even worse, lol!) So, I asked those q's, then decided to ask if I could call her sometime, after work, so I wouldn't distract her at work (which I do a lot, unfortunately, accidentally.), & I wanted to call to know her better, become better friends, (but I ALSO hope to date her, if she gets single, & wants to. Up to her). She said no to the number, her boyfriend would be jealous, but a sec later OFFERED her email. Interest? Nice? Wants me to call? Not? What?
What does it mean when a girl with a boyfriend gives you her email instead of number?
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