This site explains the different terms...
It is not rare to see novice growers wondering what IBL, BX or S1 mean. While they may seem useless, these acronyms give plenty of information when we know their meaning. In this post we tell you some basic concepts of cannabis genetics so you can fine-tune your choice when purchasing seeds.
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But basically a BX is a back cross, meaning a parent and a child were crossed. An Fx is a generational cross, meaning siblings are mated. An F1 is first gen, an F2 is a cross between siblings of the next gen, and so on. By the time a breeder gets to an F3, most genetic variation between the seeds is eliminated and each seed will produce very similar plants.