The eplex-library interfaces to an external simplex/MIP solver which might require a license to run (e.g. CPLEX, XPRESS-MP). Certain versions of these external solvers use a challenge- response licensing system. Only in such cases is this predicate needed. lp_get_license_challenge/1 returns a value which is used to compute a license key. This key is then passed into lp_get_license/2. For the details of this procedure you will need to consult the documentation of the solver vendor.
Be aware that the magic formula supplied by the vendor is usually designed to be performed in a language like C, and may rely on the behaviour of overflowing with 32 bit integers. In ECLiPSe, no overflow will occur because of bignums, and this has to be taken into account if the magic formula is calculated in ECLiPSe.
If you have a development license of CPLEX or XPRESS-MP, this predicate does not need to be called. If called anyway, it fails.
?- ( lp_get_license_challenge(Challenge) -> magic_formula(Challenge, Response), lp_get_license(RuntimeLicenseString, Response) ; lp_get_license(DevelopmentLicensePath, 0) ).