If a tag compression and exception are disabled, you will get all the values from the data source without filtering, in this case Zero and Span are not used. However I wouldn't call Zero and Span useless, they are still useful attributes and should be properly configured for tags. One case I can think of is if you make a trend of a tag in ProcessBook, it will use the Zero and Span for the scale of the display.
Some objects of PI Coresight will set up the scale considering the Zero and Span of the PI Point as well.
With Exception and Compression "Zero" and "Span" are used to calculate exdev and comdev based on exdevpercent respectively compdevpercent when e.g. using PI SMT Tag Builder add-in for Microsoft Excel. There is other usage of "Zero" and "Span" e.g. within a ProcessBook Trend, when a trace is set up to receive "Min" and / or "Max" from "Database".
"Zero" and "Span" are just the attributes of PI points. In spite of opening Exception and Compression, all the value of one point outside the range which defined by "Zero" and "Span" will not be filtered, unless your point type is Float16. However, this 2 attributes will be very useful for development. We could use these as the arguement in our coding, when we need to filter out some data transferred to our application topped on PI system.
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