No. FlexWhere can be used without FlexWhere for Desktop. (formerly TrayApp). FlexWhere for Desktop does make it much easier for employee and more accurate for statistics.
Without the TrayApp, employees must reserve a place via their smartphone. That is the only indication that the workspot is busy. This requires discipline from your employees when making reservations and canceling them if necessary.
Even with the TrayApp, employees still reserve a workspot via the mobile application.
With fixed PCs and docking stations, the TrayApp automatically registers who logs in and when. An employee with a laptop without a docking station clicks on the occupied workspot via the TrayApp on the floor plan.