Transmission of the load form the structure to the underlying soil  is the basic function of a footing or a foundation. The suitable footing that is needed for a particular construction depends on the depth where the bearing strata lies, of the soil condition and the type of superstructure.
When it happens or is needed for two or more columns in a straight line to be carried on a single spread footing, then that means combined footing. When each column has isolated footings that choice is more economical. More about in the link bellow.

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