In the "Tag to log" selection area, simply choose any existing tag from your project.  If that tag has multiple values which could be logged, you will have a choice in the "Variable to log" selection.  In most cases, the variable will simply be "value".

The "Enable" window allows you to enable or disable logging of the selected value by entering a 1 for Enable and a 0 for disable. You also have the ability to toggle whether logging is enabled based on a script expression, or by selecting a tag: either the same tag you are logging the data values for, or any other.  This provides you with the option of either logging continuously or logging only when a condition is met such as when an alarm tag is activated.

An SQL Logger Group must be selected.  SQL Logger tags contribute their values to SQL Logger Group tags which then manage the actual output of the data to a database.

If (and only if) the associated SQL Logger Group was set to trigger on data changes, it has essentially passed control to this tag to cause data writes to occur.  If this is the case, then the Deadband field will be enabled and you will have the option of indicating how much the value must change by before another value will be written to the database.