Question: Recently I received this email from a client with reference to a purchase order:

> 
> If you have received I will have to reinstate the lines on the po and then you need to grn.
> 
> 
> 

I can't figure out what the meaning could be.

Answer: 
GRN means [Goods Received Note](https://www.collinsdictionary.com/us/dictionary/english/goods-received-note) (sometimes called Goods Receipt Note). The goods receipt note is an internal document. This record is used to confirm all goods have been received and often compared to a purchase order before payment is issued.

The client wants you to *send* a GRN which will describe each item and the quantity of each item received. 

The client can use it to raise any issues, update their stock records or use it to match against the original purchase order and supplier invoice.
Not sure about the phrase "grn" as a whole, but perhaps the "rn" portion stands for "reference number?"