influx_s and influx_i are programs designed for flux and metabolite concentration estimation based on labeling data using stable isotopes (essentially ¹³C but combination of multiple isotopes like ²H, ¹³C, ¹⁵N, ... is also possible). influx_s works with stationary data while the influx_i is able to simulate instationary labeling (hence the _i in the name). The both work in metabolically stationary context. The whole project is referred as influx_si. Note also that the term influx_si is used in contexts where influx_s and influx_i are interchangeable.
For installation instructions and more details, you can check the doc online.