Let's say you have a routing-instance named rte-inst-1
When you decide to edit it you would type:
user@host# edit routing-instances rte-inst-1
As you type you could use either the Tab Key or the Space Bar to complete the entering in of the command <edit> and the command <routing-instances>
You could complete the entering in the the instance name <rte-inst-1> using the Tab Key. So typing in r<Tab Key> would fill in the rest of the characters for you. As was said in the prior post: Tab handles commands and variables, Space Bar commands only.
Hope that helps explain 🙂