The differences between dependency tags

I am new to Robotic Operating System, so this can be fundamental question. When I was investigating dependency tags of `package.xml` file of my newly-created package, I faced two dependency tags that I could not resolve. According to the researches that I made, if a file from the package A is required to build our newly-created package, then package A is specified in the dependency tags like this: ` A `. This sounds senseful. However, I could not find out following dependency tags: `` `` When I created my package, I used the following command: `catkin_create_pkg beginner_tutorials std_msgs rospy roscpp` These three packages are also specified in two tags mentioned above. Is there anyone who knows what these dependency tags mean ?