In Guitar Hero: World Tour, what does the string between slider notes mean?

In Guitar Hero: World Tour, I've noticed there are two different kinds of notes that are designed to be used specifically with the slider touch pad on the guitar.

Both types of notes are hollow-ish see-through, but sometimes there is kind of a purple string attaching strings of notes together and sometimes not. What's the string for?

These are called Slider Gems, it means you can play these notes by sliding your finger between them on the slide pad instead of tapping each note.

From the WikiPedia page:

Guitar tracks feature notes connected by a semi-transparent purple line [...] called "Slider Gems"; the player can play these notes by sliding their fingers up and down the touch pad or by tapping the fret buttons without strumming.