Authorisation form to represent dealer may use before appealate authority

For GST

An appeal to the Appellate Authority under sub-section (1) of section 107 shall be filed in FORM GST APL-01. 

For Income Tax

you are aggrieved by an order of an Assessing Officer (AO), you can file an appeal against the same before the Joint Commissioner (Appeals) or Commissioner of Income Tax (Appeals) by submitting duly filled Form 35 online on the e-Filing portal.

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