SDF format Information

Continuing the discussion from Urdf and sdf - Whan to use which?:
can you pls explain what are close chains? and information about sdf format and the tags used.

HI:

Closed chains are kinematic chains that have several branches that then reconect again. For example, the wheels of a tank are closed chains. a tensegrity robot has closed chains, car seeing mechanism tend to be also closed chains…

Information about sdf format you have it here: SDF
Here you can find all the tags used.

If you are only going to use Gazebo, sdf are enough, and the ofitial format used. If you are going to work with robots in ROS and other systems, URDF is the way to go.

thank you its clear now