The Tax Identification Number (TIN) is required for tax reporting purposes and you are required to fill this in during registration if you are a non-Malaysian citizen. This is for FATCA and CRS reporting requirements. You will be able to find your TIN on your tax returns form.
For definitions of FATCA and CRS, please refer to the subsequent questions within this category.
(Refer to: What is FATCA? and What is CRS?)