Concrete types are non-abstract classes. Often when using "Find Usages" it is sufficient to get a list of those 
subtypes that can occur as object types at runtime (i.e. that are instantiable). Having to use "Find All Subtypes" is 
inconvenient when there are many sub-interfaces and/or abstract subclasses.

Alternatively, add three checkboxes for sub-interfaces, abstract subclasses and concrete subclasses, which can be 
combined with "Find All Subtypes" and "Find Direct Subtypes Only". This would be most flexible. For example sometimes 
it can be useful to find all/only direct sub-interfaces and/or abstract subclasses.