Turing machine with k-dimensional tape or k-regular tree

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The statement I read is " In a k-dimensional tape, cells corresponds to elements of free commutative group of k generators. s. There are 2k shifts, which correspond to addition of a generator or its negative.In a k-regular tree, the cells correspond to the elements of a free group on k generators. There are 2k shifts, which correspond to premultiplication by a generator or its inverse. " It is cited from paper Relations Among Complexity Measures by NICHOLAS PIPPENGER . Could someone please explain this. (I don't have good background in abstract algebra , just know some definitions)