Calel,
You need to improve your Google search skills
I searched on "FLIR E40 PDF" and "FLIR E40bx PDF" and found the attached sales brochures in seconds
I will leave it to you to compare specifications, but basically it is a difference in temperature measurement range. The "E40bx" was intended to be used for building thermography and was a little bit cheaper than the E40.
And before you ask, yes the E40bx can be upgraded to the E60bx specification and beyond. The temperature range restriction remains however. There is a way to access the temperature range settings using the service menu found on Mk1 models but not on the Mk2. The Mk2 Exx series cameras have MSX capability but the service menu was deleted from their firmware build.
There is no guarantee that a particular E40bx camera will have the required calibration tables in it to enable a greater temperature measurement range, even if enabled in the Service menu.
Fraser