sharepoint online – Task list view permissions by department

I assume a user can only have 1 department? or he/she is only a member of 1 department?

you could just filter the view by current user [me], and add the department to the view,

in that case, he can only see his task, he only have 1 department anyways. The list should be something like

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This is a concept that I assume would work, let me know if it helps atleast.

Happy SharePointing!