The Databases - Data File Group About to Grow alarm is raised when a file in the disk group is about to grow. You can use this information to ensure the time the file resizes is suitable; for example at a time of off-peak activity.
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The Databases - Dynamic Data File Group Growths Remaining and Databases - Data File Group Space Used alarms are raised when the space allocated to a disk group is about to run out.

This Databases - Data File Group About to Grow alarm is raised before a file is about to grow so you can ensure the time the file resizes is suitable; for example at a time of off-peak activity. It is based on a ratio between FixedDataUsed and TotalDB calculated on the FileGroup procedure.