Which Invoice issued??

Sir, I have doubt regarding invoice.

Our client is SEZ Tax payer in GST and his business is hotel industry located at mundra. He supply service to others SEZ units which are located at Mundra.These supply are Exempt under GST. Then our client have issue TAX INVOICE OR BILL OF SUPPLY OR EXPORT INVOICE to another SEZ units.. It is doubtful to issue invoice. We have also LUT.

Kindly suggest... And also give rule or other things for convey to client..
Thank you in advance..