Some inventory systems (ex. Compeat, Restaurant 365) export either items or GL summaries.
For these systems, invoice line items that are set to export line items do not need to have GLs assigned since GL summaries are not exported.
If the "Export Line Items" setting is off, you are required to assign GL accounts to line items.
You can choose whether to export line items or GL summaries to by toggling this checkbox in the Vendor Center: