Repti-Bark and Sand for substrate

[Wrath]

What kind of snakes and lizards use sand? My bf asked me today, if my Rosy Boa comes from places with sand, why can't I have sand in his enclosure. I did not know why, I just use aspen. Can anyone shed some light?

Also, I went to buy the substrate for my sudan lizard, and all they had that was suitable was repti-bark, which is fir wood chips. The package says it's for high humidity, but my lizard is lower humidity. What kind of wood chips should I use? I'm probably going to have to get something online, because that's all they sell around here.