Retail Invoice means when vendor issue a bill to some ultimate user of product or services. There may be Tax invoice to ultimate user but user of product or service do not take any credit of tax.
Whereas Tax Invoice issued for the purpose of statutory liability to the government and shows that tax amount mentioned on the bill is only liability of the vendor to the government which is not shown in the retail invoice.
Moreover a user of the product or service take credit of the tax paid with the product of services.
There must be tax invoice if product or services are in tax purview but retail invoice is not mandatory. One can sale only on the basis of the tax invoice instead issue of retails invoice.