Users often set up a non-stock line type in Line Type Constants (P40205). Line Type is a required value for each Purchase Order detail line. Furthermore, it determines how a line is processed through the Purchasing system via Order Activity Rules. In P40205 for a Non-Stock line, the GL Class is setup in G/L Offset field or in Item Master (P4101) in G/L Class field.
Occasionally, in P4310 (Purchase Order Entry), users enter a line for a non-stock item that has a different GL Class code setup in the item master but the value of GL Class code defaults from Non-Stock Line Type setup in (P40205) instead. This can occur when in Line Type Constant user did not select or check Edit Item Master for Non-Stock Item in line type constant. Once the Edit Item Master for Non-Stock Item is checked then G/L Offset value from P4101 will default into purchase order entry instead of Line Type setup.
It should be noted that the info above information was taken from Line Types in Procurement (P40205 / Inventory Interface) [Video] (Doc ID 625509.1). This quick reference guide contains a lot of other information that can be helpful with regard to Line Type setup in procurement module. Of special interest is a link to a recording “Where Does The G/L Class Code Default From” in the middle of the document. The video contains detailed information about GL Class code for sales order detail lines as well as purchase order detail lines.