If you are trying to include the customer's PO, <Order Number> should work. Here is the variable info from the on-line help:
Subject Text
Subject Text is the wording that will appear in the Subject of an e-mail message or in the Regarding area on a fax cover sheet that accompanies documents sent to the customer. The text defaults from Subject Text on Quick Send Setup (21.951.00) in the Shared Information module.
You can specify the following variables within the subject text to personalize each e-mail message or fax:
• <Customer Name>
• <Reference Number>
• <Document Amount>
• <Company Name>
• <Order Number>
• <Shipper ID>
• <Ship Date>
• <Statement Date>
Each variable must begin with a less-than symbol (<) and end with a greater-than symbol (>). Also, the first letter of each word in the variable name must be capitalized and the rest of each word must be in lowercase letters.
Example: You enter a variable to customize the default subject text for invoice document types: “Current invoice from <Company Name>”. If the company name is Contoso, Ltd, the subject text for the e-mail message or fax cover page reads, “Current invoice from Contoso, Ltd”.
Body Text
Body Text is the wording that will appear in the body of an e-mail message or on the fax cover sheet in the Notes area. The text defaults from Body Text on Quick Send Setup (21.951.00) in the Shared Information module.
You can specify the following variables within the body text to personalize each e-mail message or fax:
<Customer Name>
<Reference Number>
<Document Amount>
<Company Name>
<Order Number>
<Shipper ID>
<Ship Date>
<Statement Date>
Each variable must begin with a less-than symbol (<) and end with a greater-than symbol (>). Also, the first letter of each word in the variable name must be capitalized and the rest of each word must be in lowercase letters.
Example: You enter a variable to personalize the body text for invoice document types: “Here is an invoice for <Customer Name>”. When you send an invoice electronically to the customer, Kim Abercrombie, the e-mail message or fax cover page body text reads, “Here is an invoice for Kim Abercrombie”.