Sell without updating HSN in gst registration certificate

Under the GST registration, the taxpayers is required to mention the top 5 HSN/SAC codes. 

So, if a taxpayers can use HSN or SAC which is not mentioned in GST registration certificate. 

So a registered tax payer can sell goods whose HSN is not mentioned on the GST registration certificate

What is the Notification Number for  (Yes.Its not mandatory for few enterprises..Initially the HSN Code was printing on GST certificate. So we may not expect HSN code on the RC) ----- form GST dept

If we have issue an invoice with a HSN code which is not mentioned in RC (at that time lets take only 2 was there in RC)- then in that case what will be the ideal scenerio.

Any notification is there to support this?