Hi dongmei,
SNP Production horizon - After planning run (SNP and CTM)planned order generate after this horizon days.
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Extend Production horizon - Time period, during which Supply Network Planning (SNP) does not plan production; that is, does not create any SNP planned orders. SNP moves production to the first day after this horizon. This horizon is an extension of the SNP production horizon. However, in contrast to the SNP production horizon, the user can manually create SNP planned orders in this extended horizon in interactive Supply Network Planning.
Refer : Difference between SNP horizon, SNP production ... | SCN