Handle scale is the full term for a scale: one of the two slabs of material fastened to the sides of a knife tang or liners to form the grip. Spec sheets use both words interchangeably.
When a listing says "black G10 handle" it means the scales are G10. When it says "jigged bone handles", plural, it is naming both scales. Neither phrasing tells you anything about what is underneath, which is why liners are usually specified separately.
On a full tang fixed blade, the scales are visibly separate pieces with steel showing between them. On a folder they wrap the liners.
A well-shaped scale in cheap material out-grips a badly shaped one in an expensive material every time. Look for contouring rather than flat slabs, and for a shape that fills the palm rather than sitting under it.
Very thin scales save weight and lose comfort quickly under hard use, because the pressure of gripping concentrates on a small area of your hand.