This is simply the name of the steel works.
But...some project partners consider this information as strategic and confidential. Thus, a conversion table is mandatory to keep track of the steel works information while not disclosing it. This is why some steel works are named e. g. S1, S2, etc. If you feel this is your need too, you are free to do the same, even if we do not recommend it. In that case, please call your suppliers 1, 2, etc., in a unique way, and add your company's prefix, i. e. X1, X2, etc. Then you and only you know who is who, and can translate it.