T = Taxable / Not Taxable for the selected invoice item. (Yes this allows you to mix taxable and non-taxable items on one invoice)
W = Work Required / Work Not Required

The next 3 are mutually exclusive i.e. typing one and enter changes the line item status between them.

P = Purchased
R = Return
S = Special Order

*  = Misc Item

12.50 = Change price to $12.50
.10 = Discount off of retail price (Not Current Price)
..10  = Additional 10% discount off of current price

Barcode/UPC Number = Add that item to the invoice

Any integer = Quantity (negative numbers also change line item to return)

Last/Company Name = Pick that Customer