You would need to look into what type of the interface your tag is coming from. However in case of RDBMS interface, it is written as below:
If any PI point is removed from the Interface while the Interface is running (including setting the scan attribute to 0), SCAN OFF will be written to the PI point regardless of the value of the Scan attribute. Two examples of actions that would remove a PI point from an interface are to change the point source or set the scan attribute to 0. If an interface specific attribute is changed that causes the tag to be rejected by the Interface, SCAN OFF will be written to the PI point.
Other common interfaces such as OPC:
You can enable scan attribute to 1 while the interface is running. But it will take some time for interfaces to realize that point is now added for scan and therefore actively search in the HMI.
Hope this helps.
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Scan Off: The tag attribute Scan has the default value of 1, which indicates that the interface should be able to collect data for the point. Setting the Scan attribute to 0 turns off data collection for that point. If the Scan attribute is 0 when the interface starts, the Interface will not load the point (SCAN OFF will not be written) and the point will not get any data updates. If the user changes the Scan attribute from 1 to 0, while the interface is running, the Interface writes SCAN OFF only when it detects that change. This might take up to 2 minutes.There is one other situation, which is independent of the Scan attribute, where UniInt will write SCAN OFF to a PI point. If a point that is currently loaded by the interface is edited so that the point is no longer valid for the interface, the point will be removed from the interface, and SCAN OFF will be written to the point.
All that have answered this question, thank you very much and, a special thanks goes to Jinmo Yi and Eugene Lee.