Confused on different make commands for PX4

What are the differences when doing each of these different make commands

make px4_sitl
px4_sitl_default
px4_sitl_rtps

and then when you add say gazebo, does passing gazebo also alter what gets built?

make px4_sitl gazebo
make px4_sitl_default gazebo
make px4_sitl_rtps gazebo

Thanks