What's the difference between a sleeping bag liner and a travel sheet?
Shape (are travel sheets only available in one shape)? Fabric? Resistance?
I doubt there is an "industry standard" of what each of those mean, but in general I have noticed:
Sleeping bag liners tend to be narrower, and come in mummy-bag and regular:
"Travel sheets" I have seen are bigger, square, and have slits down 1/3 to 1/2 the side for easy in and out.
Using a travel sheet as a sleeping bag liner becomes an issue if it is significantly larger than your bag. The excess fabric tends to tangle up in your legs.
As for fabric, you will find both in all types: cotton, silk, synthetic, flannel... though sleeping bag liners often have more light-weight options like silk since you will likely be carrying it into the back-country, since they are a great way to extend the length of time between washings.
Not sure what you mean by "resistance."