: What are these?
: What's the difference in them?
: When is one used in place of the other?

Functional difference is that for a given weight the s-glass will give more before breaking, fractures in the wood won't travel as far and the wood will be held more intact when it fractures. It's also more abrasion resistant. 4oz s-glass won't replace 6oz e-glass.

Some kinds of e-glass can be almost as strong as s-glass, I forget the name, it's not woven cloth but the fibers are layed flat.

You use it when you want to spend more money for a bit more durability and don't mind that it deosn't wet out as clearly. You can't use 4oz s-glass as a replacement for 6oz e-glass if impacts are a concern, but if you were going to use 4oz e-glass for an ultra light build then 4oz s-glass would give you bit tougher laminate.

theoretically it's higher tensile strength would make more sense for the interior of a laminate but for the bottom of the hull it's slightly greater abrasion resistance is worthwhile,,but that's going to be splitting hairs. My experience is that external dings in 4oz e-glass that dented the underlying wood would fracture with waterstaining more readily than 4oz s-glass where dings dented the underlying wood. This characteristic was similar to some test panels I made where two panels with 4oz sglass and eglass were bent to failure . The e-glass panels shattered with exposed wood, the 4oz sglass panel bent into a C with whitened threads and cracking of the wood but the panel didn't break through. But with heavier 6oz glass the panels both shattered albeit at much higher efforts but the e-glass fracture was messier and the s-glass confined to a smaller area.

In puncture tests the difference was also noticable. Whether stabbing with a screw driver or dropping a concrete breaker bar onto the panels the fractures in the e-glass extended further out from the hole along the direction of the fibers. In the s-glass the fractures extended a shorter distance with the white stress marks in the glass in a more compact random patter. For example if I dropped the 20lb bar onto a 6"x6" panel glassed with 6oz it would punch through either one but the damage in the wood extended about twice the distance in the e-glass panel.

basically neater application with more glass is heavier and more durable than light glass,even if it's s-glass.