This loose term is more frequently seen used within professional horticulture but simply means the soft things that grow ( i.e. plants) and the soil or compost they grow in. So it includes trees, shrubs, hardy perennials, grass, etc.. It generally doesn’t include vegetables grown purely for consumption, there are a number of very ornamental vegetable that would then be soft landscaping, as commercially a landscaper would not normally be involved in vegetable growing, just providing space for their cultivation in a garden. Commercially vegetable growing would be something undertaken by a market gardener which has nothing to do with landscaping.