When saving a scala source file that resides in a project, eclim will update that source file in Eclipse and will report any validation errors found. Any errors will be placed in the current window's location list (:help location-list) and the corresponding lines in the source file will be marked via Vim's :sign functionality with '>>' markers in the left margin.
Automatic validation of scala source files can be disabled via the g:EclimScalaValidate variable (described below). If you choose to disable automatic validation, you can still use the :Validate command to manually validate the current file.