An example might make clear what Jazzns means by "erosion that causes sediment to go away." A mountainous region drained by rivers will be an area of net erosion. The mountains are worn down by weathering. The products of erosion accumulate in the valleys and eventually make it to the rivers and are carried downstream and out of the region. Mountainous regions are areas of net erosion.
Mountains are areas of erosion yes, but not necessarily 'net erosion'. In that case, there would never be any mountain ranges. Only when erosion exceeds uplift is there net erosion.
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