From Aptana Development

When adding a plugin, which feature you add it to depends on what the intended audience is for the plugin

com.aptana.ide.feature

All Studio users, both plugin and standalone. An example here is the database plugin which all users of Studio should get.

com.aptana.ide.feature.rcp

All Studio standalone users. These would be items that don't apply to Eclipse plugin users. Generally, very few things fall in this camp--the only items currently are plugins which are specific to running studio as a product, not related to any core functionality.

com.aptana.ide.feature.rcp.wrapper

Only new installs of Studio standalone. An example here is SVN. We only want brand-new installs of Studio to include the SVN plugin so as not to conflict with existing installs, either by users with Studio in Eclipse, or by standalone users who had previously installed Subclipse or Subversive.