If an object is accessed with a different coded character set identifier (CCSID) than was previously scanned for that object, a scan would be triggered.
An example of this scan is when a file with data stored in CCSID 819 is opened in CCSID 1200, and scanned successfully. As long as the file's data is not changed, then every time that file is opened in CCSID 1200, a scan is not triggered. However, if that file is opened in a different CCSID, for example, 37, a scan is triggered for that CCSID 37. If that scan is also successful, then any subsequent accesses with CCSID 1200 and 37 will not trigger an additional scan.
Only two CCSIDs and one binary indication are kept in an effort to minimize data stored on the system. If you typically access the same object with many different CCSIDs, then this can cause a lot of additional scanning to occur.