What is meant by the phrase units started and completed? Why is this phrase more closely associated with the first-in, first-out method of process costing than with the weighted average method?

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Description
What is meant by the phrase units started and completed? Why is this phrase more
closely associated with the first-in, first-out method of process costing than with the
weighted average method?
Units Started
Units Started are the units that enter the production department during an
accounting period.
Units Completed
Units completed are the units that have been physically...