The setting changes the DISPLAY only. Values are always stored and calculated in the metric system, so switching there and back spoils nothing and does not accumulate rounding errors.
What changes: the solution and air temperatures (°C or °F), the box dimensions, the grid step and the working area of the cover (millimetres or inches), and also the look of dates and times.
The date format follows the unit system, not the interface language: metric gives 07.08.2026 and a twenty-four hour clock, imperial gives 08/07/2026 and a clock with AM and PM.
What does NOT change: the cell size marking. 40 mm, 3", 4" are written the same way in both systems - this is the name of a standard size under which net cups are sold all over the world, not a measurement. Showing «1.57 inches» instead would get in the way rather than help.
TDS and EC do not change either: ppm and µS are the same in both systems.