第二，订货至交货的时间是可以提前知道的，或者可以提前估算出来。
The second is that the lead times are known, or can be estimated, in advance.
后者是计划到什么时间，是由已知需求和生产中订货至交货的时间所决定的。
The `planning horizon' is how far to plan forward, and is determined by how far ahead demand is known and by the lead times through the operation.
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