Hi Jinkai,
If you have only one level you don't need to provide the level2_code column.
Leaving empty values for the second level means that you are defining the coordinates (and information) related only to the first level only.
When you will use an expression like idm:samplingPointCoordinate(FIRST_LEVEL_VALUE) you will access the coordinate in the row of the sampling point data having level1_code column equal to the specified one and empty values for the levels greater than the first one; an expression like idm:samplingPointCoordinate(FIRST_LEVEL_VALUE, SECOND_LEVEL_VALUE) will give you the coordinate in the row having level1_code and level2_code columns equal to the specified values and empty values for the third level column.
It can be useful when you have a survey structure like cluster -> plot and you want to insert in the sampling point data the coordinates of the center of the clusters while there is not a plot having the same coordinate, or when you want to define other information related only to the cluster and not to the plots inside of it.
I hope it is a bit more clear now, many thanks.
Open Foris Team