Additions not being allowed to Fixed Assets using NZSL depreciation method

Hi MYOB Community

Does anyone understand and can explain to me why the NZSL depreciation method has been created in MYOB Advanced to NOT allow additions or deductions?  I would like to know the piece of legislation that this restriction relates to.  Or has it been adopted for NZSL because AU Prime Cost required it and it was thus assumed NZ was the same???  Just after any thoughts and clarifications on this..  Thanks