I know at my boyfriend's chapter of Sigma Chi, Lavaliering a girl was a chapter decision. The brother may bring up the idea at chapter meeting, but it was a vote amongst the brotherhood whether or not they believed the girl was worthy of wearing their letters. Being allowed to wear the necklace and lettered shirts meant you were important to the brothers and they felt you represented the principles of the fraternity.
His chapter tradition usually included dinner with the brother and girlfriend, then going back to the chapter house and having the brothers circle around the couple and sing the Sweetheart of Sigma Chi song.

In contrast, from what I know, Pinning is something that is personal between a brother and his girlfriend only. A pre-cursor to engagement and whatnot. I was lavaliered at his chapter house, then became the chapter sweetheart a year later, and when I finally moved in with him after college he gave me his badge. I wear it with pride, not only knowing I am loved very much by a Sigma Chi, but also as a Sweeheart, by the whole fraternity.

I was never in a Sorority so I never participated in a candle pass but it sounds wonderful! <3