The Apache Sling Service User Mapper Service is a global configuration, and that presents some difficulties because service user mappings are typically bundle-specific.
For example you have 2 sites deploys on one instance:
SiteA, whîch maps serviceA to userA
SiteB which maps serviceB to userB
In order to avoid having to do the mappings manually when the two sites are deployed, you add each to a configuration that is installed with your bundle. The problem then is that if you deploy siteA and then siteB, the siteA mapping will be overwritten by the siteB mapping! Basically, whichever bundle you deploy last will take precedence.
In order to solve this, the Apache Sling Service User Mapper Service Amendment (which not a configuration but a configuration factory) allows you to add one configuration per bundle. This way you can deploy both sites in whatever order and you'll still retain both configs 🙂 If you have a fresh instance and install ACS Commons, you can see this in practice:
Nothing in Apache Sling Service User Mapper Service:
All mappings in an Amendment:
If I were you, I would also use an Amendment, even if you only have one bundle to deploy... you never know what the future will bring!