Hi,
Normally when a material is created in sap , this material can not be made distinguished as a normal material which is for normal procurement with a normal vendor or for a STO Plant vendor .for a STO PO
But however you can have the following steps . Create a MMPP status which will not have controlling properties ( blank in its properties at SPRO ) assign it to the material and write a small pice of code that when a material which is having this status will have to get used with a Plant vendor as a error message in the PR function module where it cheks the MMPP status of the material .
However at PR stage threre is no STO PR kind of concept . Once you convert the PR into PO at PO ME21N , when you assign the vendor , here comes the normal PO / STO PO difference . For such materials which are purely for STO just create the source list with the STO vendor exclusively for this material as fixed and do not add any other sources to the source list .Keep it exclusive and use the functionality source determination so thaO system will take care of the STO vendor assignment and STO PRs only will get generated .
Create another material number for the same material which will have the same properties for the normal purchase.
Regards