A519 = Seamless Carbon and Alloy Steel Mechanical Tubing. "4130" refers to 4130 alloy steel, i.e., the material it is made of.
"Tubing" is called out to OD and wall thickness (an even increment value). 6" x 1/4" round tubing will be 6" OD x 5.500" ID.
"Pipe" is called out as nominal pipe size (ID) and "schedule" for wall thickness. 6" Schedule 40 pipe will be 6.625" OD with 0.280" wall, and thus 6.065" ID.
Mechanical tubing is used for all sorts of general fabricating and machining. For instance, a race car frame, such as a NASCAR vehicle, is made from A519 4130 tubing that has been bent and welded together. (Well, they were in the 90's.) Many round & hollow machinery parts which are turned to shape on a lathe are made from 4130 tubing. A seat ring in a ball valve for instance.