Dear Ravi.,


It's not a critical.

Just be SAME GSTIN then no need to prepare Tax invoices for transfer of Stock....
But,
Different GSTIN then it's treated as Supply of Goods and Services and GST will Chargeable / applicable...

As per Your
1st Query - The Transfer will be made on TAX INVOICE and Also it's treated a Sale Proceeds...

The same as 2nd query with IGST.