You guys are all correct to one degree or another about what makes a tool carrier a tool carrier, But, you're mising the one big reason they have the parrallel, or retractable linkage.
It's so it won't interfere with the tool when the attachment is in full rollback. The Bell crank on the Z-bar machine comes forward, into the back center of the attachment when you roll it back. There for you are very limited in what you can mount on the machine.
The Parrallel Linkage machines, the Cat Uni-booms, The John Deere Powerllels, or Volvo Style Linkages have no bell crank to interfere with the attachment, so you can mount anything on them. Hence the name Tool Carrier.
Simple. They can just carry a larger variety of atttachments.