They speak of the place that is best to get the sample iliac crest which is the hip area. That is where mine has been done. The go into the bone and an aspiration of the fluid and many also get the 2nd which is the bone.
Looking for what a bone marrow biopsy entails, i found this post on google from Mayo Clinic. A site you can trust.

*Bone marrow has a fluid portion and a more solid portion. In bone marrow aspiration, a needle is used to withdraw a sample of the fluid portion. In bone marrow biopsy, a needle is used to withdraw a sample of the solid portion.