This is simply the name of the part producer. This information is only interesting to supplier companies dealing with heat treatment of someone else's parts. The default value is own production.
Some project partners might consider this information as strategic and confidential. Thus, a conversion table is mandatory to keep track of the part producer information while not disclosing it. This is why some part producers are named e. g. M1, M2, 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.
Whoever the producer is, it can always be interesting to keep track of which production bach a part belongs to.
Please notice that parts coming from different production batches but heat treated in the same charge should be considered as different cases.