Hi wssviper,
It sounds like the one you have is 5 sided (doesn’t fully enclose the mattress and have a zipper) which is more like a mattress protector or mattress pad than a mattress encasement (which is used for allergies and bedbugs).
The top is quilted and probably uses polyester fibers so it would be thicker and add some surface softness to the mattress but because it’s thicker it would also interfere to some degree with the compression of the latex and its ability to contour to your body shape. It has a waterproof membrane so it also performs the function of a mattress protector unlike the one that is double quilted which would probably have a little more padding (again polyester fiber) but lacks the waterproof layer.
Out of the mattress protectors it would be closest to the “thin membrane type” in terms of its waterproofing method (they both use a semi breathable waterproof membrane) but it also has padding which the thin membrane types don’t have so it would be more like one of the fiberbed mattress pads mentioned here (with a semi breathable / waterproof membrane) which generally use a lower quality polyester fiber than the “down alternative” versions which use a similar but higher quality and slicker and more “slippery” polyester fiber which feels more like down.
There really isn’t a clear “boundary” in terms of defining which is which (protector vs mattress pad) because some of the wool protectors also use wool as a quilting material (which provides breathability and temperature control and is water resistant) which can also have a bigger effect on how the latex underneath it contours and compresses but in general protectors are designed primarily to protect the mattress and have less effect on the feel of the mattress while mattress pads are designed to add a little bit of surface padding and softness to the mattress and affect the “feel” more.
Phoenix