§25.25 Weight limits.
(b) Minimum weight. The minimum weight (the lowest weight at which compliance with each applicable requirement of this part is shown) must be established so that it is not less than—
(1) The lowest weight selected by the applicant;
(2) The design minimum weight (the lowest weight at which compliance with each structural loading condition of this part is shown); or
(3) The lowest weight at which compliance with each applicable flight requirement is shown.
What JT says seems reasonable to me as per 25.25 (b)(3) above. During my initial type rating training on the old Westwind 1124 I recall one of the instructors talking about how they were ferried to the USA from Israel "green" with no interior, minimal equipment installed and just enough lead ballast secured to the floor up forward to bring it up to minimum flight weight and within allowable CG at the end of the usable fuel supply. A typically equipped 1124 with a "corporate" pax interior is usually has a BOW about 1,000 lbs above the minimum flight weight. A few of this model were used for freight hauling and in a cargo configuration, could actually reach a weight below minimum if flown empty with very little fuel. I'm just guessing here, but I suspect the 1124 minimum flight weight was the lowest weight selected by the applicant or was simply as light as it was ever flown in testing.