SDF versions

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Gazebo version 2.2.6, Indigo

Hello, ROS Community!

I would like to ask you about SDF versions. Nowadays, I am confused what SDF version is appropriate for running .world files. The question is appeared when I got errors following gazebo tutorials. Here , the .world file used SDF version 1.5 and It gave me errors:

Error [Converter.cc:127] Unable to convert from SDF version 1.5 to 1.4
Error [parser.cc:697] XML Element[population], child of element[world] not defined in SDF. Ignoring.[world]
Error [parser.cc:688] Error reading element <world>
Error [parser.cc:348] Unable to read element <sdf>
Error:   Could not find the 'robot' element in the xml file
     at line 81 in /build/buildd/urdfdom-0.2.10+dfsg/urdf_parser/src/model.cpp
Error [parser_urdf.cc:2608] Unable to call parseURDF on robot model
Error [parser.cc:273] parse as old deprecated model file failed.

I looked other .world files that run without any problem.. The files is under SDF version 1.4.

I was surfing ROS answers and gazebo tutorials, I didnt get the right answer.. If you will look at the resource, you will see versions 1.4, 1.5, 1.6 are available. But, there are exist higher version in downloads

What the version can I use in order I will not have any errors in version discrepancy?