Difference between "_generate_messages_cpp" and "_exported_targets"

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Hello,

I am gathering all my messages in a single package (named, say, "my_msgs") to simplify package dependencies, (a well-known ROS good practice).

I realised that some packages add dependencies to my_msgs package using the line

add_dependencies(target my_msgs_generate_messages_cpp)

in the CMakeLists.txt, while others use

add_dependencies(target ${catkin_EXPORTED_TARGETS})

which is how the documentation indicates this should be done. I haven't found much relevant information about the "_generate_messages_cpp" variable, so I wanted to know: is there any difference? Is the second method preferred over the first one?