When you start a timer, you choose what the time is logged against so your labor cost is categorized correctly.
A specific line item / service task/defect: pick the task from the work order so the time and cost attach to that line. If a line item has a tracked time entry, its manual Labor Hours field becomes read-only to prevent double-counting.
General Labor: use this when the work isn't tied to a single line item. The time is logged against the work order as a whole.
Note: Labor can be tracked either under General Labor or against individual line items, but not both. Once labor has been tracked using one method, the other option will be disabled.