You're missing the entire point of Robb's plan with regards to the Green Fork battle because you are confused about the meaning of one single word. Instead of trying to understand what a military confrontation means, ignore that word you do not understand and focus on the entire conversation Robb has with Catelyn about his plans.
Here is a simple breakdown of Robb's plan:
- Take on Jaime's forces to break the seige of Riverrun.
- Keep Tywin distracted and engaged long enough to ensure he doesn't have the chance to come to Jaime's aid.
They cannot employ sneaky methods like striking the camp at night or catching Tywin unaware because Tywin's army is just too large and it's impossible to apprehend every single spy/scout Tywin would send out. The can't smash Tywin's army. The only hope Robb has is to sacrifice a small part of his army in order to keep Tywin distracted and engaged long enough to allow Robb to reach and defeat Jaime's part of the Lannister force. You don't keep an army distracted by sitting within seeing distance of them. Doing that puts a commander on high alert because it indicates that you are employing a sneaky plot. Keep in mind that Tywin thinks Robb is a greenboy who doesn't have the experience to make cunning battle schemes. It's to Robb's advantage to allow Tywin to keep thinking this for a while longer.
The Greatjon is too fearless to accomplish this type of scheme. He would go in swords-a-blazing and lose the entire part of that force. He wouldn't be able to coldly choose the small part of the force to be committed to sure death and/or captivity. The Greatjon may be fearless and not too clever, but he's still a vital part of Robb's war council and care should be taken to keep him there. So in comes Roose because he's cold and clever and calculating.
Roose does exactly as Robb planned and commanded. Roose chose the men to send in who would surely die or be taken hostage. That takes a highly cold and calculating person. We don't have the conversation that happened between Roose and Robb before he was sent off, but it's nearly certain that they discussed which part of the force Roose would send to death and hostage and, considering Roose's calculating nature, he surely advised Robb on who he should send in for the slaughter and also surely made certain that he could advise Robb to agree that none of Roose's Dreadfort men are lost.
Again, Roose does exactly as planned. He engaged (in other words, confronted) Twyin's forces just long enough to keep him distracted and he coldly chose who would be sent to death before retreat.