The purpose of sub items is to give you more description to your line items without having to create numerous line items and adding them to the price list.
The most logical use is for your sign codes. If you want to show the sign plus the legend on the ticket and invoice, instead of setting all of them up as line items you set up your signage as a sub item.
For example, most people will set up a sign line item based on size (i.e. Large Sign - > 16 sf), and add it to the price list. When creating a ticket there is a tab to select specific sign legends to be delivered, but they do not appear on the invoice to the customer. They will only see a # of large signs at a specific price.
However, some customers do want that level of detail on the invoice so if you enter in the item of Large Sign on the ticket, then select the specific sign legend using sub items, the data will concatenate on the ticket and invoice as Large Sign - Road Work Ahead.
Keep in mind while using sub items keeps you number of line items and price lists items to a manageable number, it may increase the number of pages on your invoice as well as other reports.
Sub Item Code: If you plan to use this feature in conjunction with normal Sign Codes, keep this the same as what was entered in Signage (i.e. MUTCD or alpha code).
Description: Type in the description as you want it to appear on the ticket and invoice.
Line Item: Associating it to a specific line item (i.e. large sign) is optional.