There are four functions when you go into the menu.

Set F/C depending on which you prefer. (Why isn't Kelvin an option?)

Set your Setpoint

Set your differential (this is degrees at which below the setpoint your thermostat will kick on, assuming you are in heating mode. It's the reverse for cooling).

Set H/C for heating or cooling mode (presumably you are heating).

If you are controlling the hot side of the enclosure then setting it at 90 with a 1 degree differential will make it turn on at 89 degrees and turn off at 90 degrees. Hope that helps,

JonV