Most innersprings are not suitable as a comfort layer since they are not so comfortable to sleep on directly although some can provide the "conforming ability" to help a comfort layer do its job. There is however a type of innerspring called a "microcoil" which is made by several manufacturers and which is designed specifically for use as part of a comfort layer. They are made from thinner more flexible wire, have more coils (usually over 800) and have less thickness so they can fit in a “eurotop" or "boxtop" mattress since they can not be made as thin as foam or fibers. They are typically used with a thin foam layer or quilting layer over them so you won’t feel the individual coils. As a comfort layer, they are much more pressure relieving and comfortable than sleeping directly on their more supportive innerspring cousins.
While they are more expensive than the low grade polyfoam that is typically used in the upper layers of a mattress, they are less expensive than a good quality foam like HR polyfoam, latex, or high quality memory foam. They also have a high level of resilience and point elasticity and so like latex or HR polyfoam can provide support to the recessed areas of the body that are not in direct contact with the support layers of the mattress. They are also more durable and less prone to body impressions than the lower quality foams that are so often used in mattresses.
They are typically used over innerspring support cores (probably because the same manufacturer or supplier would likely make both), and are often called “coil on coil” mattresses. The same or similar terms are also used for other types of innerspring combinations so be a little careful when you hear this term that it is actually referring to microcoils being used in the comfort layers. There is also no real reason that these couldn't be added over a polyfoam or latex core except that it may need a cushioning layer to protect the foam below it. It is rarely found with this combination however. Overall, they represent a good value option in a mid range mattress that is well worth testing as an option if they are available in your area.