As per my opinion the floating foundation is
When the soil is so soft that even friction piles will not support the building load, the final option is the use of a floating foundation making the building like a boat that obeys Archimedes’ principle—it is buoyed up by the weight of the earth displaced in creating the foundation. Floating foundations consist of flat reinforced slabs or mats or of reinforced concrete tubs with walls turned up around the edge of the mat to create a larger volume.
And where there are other combined footing can also be designed to carry the overall structure load, but the area covered by those combined footings are more than 70% then we choose to use floating or raft foundation to minimize the discrepancies in the case of combined foundation very near to each other.