RVM uses the concept of a 'ruby string' to uniquely identify each interpreter installation. Most commonly this is of the form $interpreter-$version-$distinguisher. For example 'ruby-1.9.1-p378' stands for "Ruby 1.9.1 patch level 378".
If you are developing tools which use RVM and you need a way to expand ruby selectors to their full ruby strings then you can do it like so:
rvm strings 1.8.7
This will produce output similar to:
You may also pass multiple selectors:
rvm strings 1.8.7 1.9.1 jruby
Which yields something like,
ruby-1.8.7-p249 ruby-1.9.1-p378 jruby-1.4.0
RVM tracks the latest stable patchlevels for each release. If you want to override this, for example, so you could make '1.8.7' to resolve to 'ruby-1.8.7-p302' instead of the latest patchlevel, you can edit the config/user file, which will override the settings in config/db and persist across installs and updates of rvm itself.