In Katalon 5.7.0 version you must select, at the Data File (for Excel Files), the specific sheet you want to use/refer for the Data File.
Then you can select at Data Binding the Excel File and relate column values to the Test Case variables.
So, if you want to change the sheet for a variable you must do it at Data File, then it will be aplyed for all the variables which uses the same Data File (very inflexible).
I suggest we should/could select the specific desired sheet directly at Data Binding (at Variable Biding). In the current version of Katalon it is not possible.
Currently, if the selected sheet is changed at Data File or if the sheet is deleted from the Excel File (it is a real bug I faced out recently) we have some problems of reference. So, it even seems to be more secure to select the specific sheet at Data Binding for Excel Files.
The selection of sheets at Data File would be available just for visualization purpose or to define the default sheet.