What does the 'S' and 'L' mean next to the invoice/PO statuses?

'S': The invoice has split line items. While in the grid view, the row of the invoice details will only display some of the data from the first line on the invoice. The 'S' is to indicate that there are more lines with more information which you cannot see unless you go the details view of the invoice/PO.

'L': No legal entity has been chosen for the invoice. So, the owner will need to select a legal entity before the lists for that entity will populate (vendor, account, etc.).