First of all, because linen’s flax fibers are slightly longer than those of cotton and wrapped a little tighter, linen sheets are stronger which means they will last much longer. If softness is a factor for you, you may want to consider cotton, though. Flax fibers are much rougher than cotton resulting in slightly crisper fabric. But the great thing about linen is that it gets softer after each wash, so essentially it gets better with time.
When we use the term bed linen we are generally referring to the sheets used to cover the mattress so that the sleeper is comfortable and the mattress is protected. If blankets or covers are being used a second sheet needs to be used for comfort between the sleeper and the roughness of the blankets. If a continental quilt is being used the top sheet is usually omitted.