Send All From the Start
Send All From Selected Line
Send Selected Line (Single Step)
When drip feeding long programs you might experience a tool break. When you're ready to restart DNC you wouldn't necessarily want to go right back to the start. You'd probably want to select a specific line and run DNC from that point. If you weren't sure about a specific area of your program you might want to single step through it.
During drip feeding at full speed you can hit pause and switch the mode to single step DNC - for example if you wanted to single step through an area you wanted to observe closely - and at any time switch the DNC back to full speed machining.