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  How to put the same coefficient in all the services of the same subject?

To give the same coefficient in all the services of the same subject, you have to work in multi-selection: go to the tab Grading> Grading services > Grading services (per class), select all classes on the left using Ctrl + A, select All services/All periods in the menu above the list of services, sort the list in the column Subjects by clicking above the column and then select all the services of the wanted subject by clicking on the first service then Maj + click on the last service of the series.

All that remains is to right-click on the selection and choose the command Modify the services > Coefficient.