Spec Limits for a given SPC view can be hard coded, looked up in a query and bound to the chart, or returned with the dataset and bound to the chart. These are the limits used for the corresponding Spec Limit alarms.
User Spec Limits can be used as an additional set of lines for interacting with the chart in what-if type scenarios. Perhaps you are viewing all of the items in a batch or lot, and the formal spec limits are being shown, and the batch was a very consistent and high quality run, meaning that all of the data points are close to the center line and not anywhere close to the spec limits. If tightening the spec limits could lead to selling the batch at a higher grade and price (sometimes production runs can overachieve the intended target grade), the user spec limits could be provided to the user through html/javascript to allow for an interactive trial and error approach to see what additional higher selling grades might be applicable to this batch (sort of like squeezing the spec limits down).
If you squeeze down the batch by adding the user spec limits, some of the data points may now be 'out of spec' for the proposed user specs (not the formal specification limits/alarms that might indicate throw away or scrap because of off-grade quality). With these line in the chart it will also be easy to use the interactive data points and attributes to identify some of the points that would now be out of spec for the user limits and would need to be pulled from the batch in order to meet the criteria of the higher paying product grade.
Regards,
Jeremy
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