I have some information about this. Soft tissue tumors form a large variety of tumors, most of them benign, some malignant and called sarcoma - sarcomas.
The source of these tumors is the intermediate, multi-force, and primitive mesenchymal cells, which are classified during their evolution into a tumor, into one or more cells.
Soft tissue tumors are classified according to the type of tissue that the tumor tries to develop (lipid, connective tissue, muscle, etc.). Thanks for the valuable participation @ janusface