By default SAP has already delivered some standard Alternative ID type and you could find them in this customizing:
For type ALT_NO, it is mapped to settype ISAM_O_VEH_IDS:
In order to use this type in WebClient UI, you have to: (1) assign ID type to corresponding product category:
(2) Also assign the settype ( which is bound to ID type ) to product category:
Then you can maintain this alternative ID in Web UI:
And it is also possible to search against alternative id:
Technically the alternative id is still implemented based on a concrete settype.