We had troubles in debugging the virtual URI mapping application/evaluation order (for a 
MultiSiteRegexpVirtualUriMapping)
By chance, we found out that the number of matching Regular Expression Groups might be involved in the weighing of each mapping (which was confirmed by Jürgen Brommer and a technician), and that the mapping with the maximum weight is to be applied. However, we had troubles in understanding and debugging these issues, since we could not find any logger who would report on that (with ALL log level). 

We would very much appreciate if you update the docs on this.

We found a groovy script for debugging, but unfortunately, this did not work with our MultiSite-Configuration:

https://gist.github.com/mkgl/c800a005b7ee960351fae1563d7a148f

Heaps of thanks and best wishes from the JobRad CMS/Platform team.